One cannot be a Catholic who engages in Jewish religious practices without committing mortal sin. Don’t believe me? Here is what the Church authoritatively teaches: COUNCIL OF FLORENCE (1442) — CANTATE DOMINO: “It firmly believes, professes, and teaches that the matter pertaining to the law of the Old Testament, of the Mosaic law, which are divided into ceremonies, sacred rites, sacrifices, and sacraments, because they were established to signify something in the future, although they were suited to the divine worship at that time, after our Lord’s coming had been signified by them, ceased, and the sacraments of the New Testament began; and that whoever, even after the passion, placed hope in these matters of the law and submitted himself to them as necessary for salvation, as if faith in Christ could not save without them, sinned mortally. Yet it does not deny that after the passion of Christ up to the promulgation of the Gospel they could have been observed until they were believed to be in no way necessary for salvation; but after the promulgation of the Gospel it asserts that they cannot be observed without the loss of eternal salvation. All, therefore, who after that time observe circumcision and the Sabbath and the other requirements of the law, it declares alien to the Christian faith and not in the least fit to participate in eternal salvation, unless someday they recover from these errors.” ^That is Dogma. SECOND COUNCIL OF NICAEA (787) — CANON 8: “ That Hebrews ought not to be received unless they have been converted in sincerity of heart. Since certain, erring in the superstitions of the Hebrews, have thought to mock at Christ our God, and feigning to be converted to the religion of Christ do deny him, and in private and secretly keep the Sabbath and observe other Jewish customs, we decree that such persons be not received to communion, nor to prayers, nor into the Church; but let them be openly Hebrews according to their religion, and let them not bring their children to baptism, nor purchase or possess a slave. But if any of them, out of a sincere heart and in faith, is converted and makes profession with his whole heart, setting at naught their customs and observances, and so that others may be convinced and converted, such an one is to be received and baptized, and his children likewise; and let them be taught to take care to hold aloof from the ordinances of the Hebrews. But if they will not do this, let them in no way be received.” FOURTH LATERAN COUNCIL (1215) — CANON 70: “ Jewish converts may not retain their old rite Certain people who have come voluntarily to the waters of sacred baptism, as we learnt, do not wholly cast off the old person in order to put on the new more perfectly. For, in keeping remnants of their former rite, they upset the decorum of the christian religion by such a mixing. Since it is written, cursed is he who enters the land by two paths, and a garment that is woven from linen and wool together should not be put on, we therefore decree that such people shall be wholly prevented by the prelates of churches from observing their old rite, so that those who freely offered themselves to the christian religion may be kept to its observance by a salutary and necessary coercion. For it is a lesser evil not to know the Lord's way than to go back on it after having known it.” SYNOD OF LAODICEA (FOURTH CENTURY) — CANONS 29, 37, 38: “29. Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ. 37. It is not lawful to receive portions sent from the feasts of Jews or heretics, nor to feast together with them. 38. It is not lawful to receive unleavened bread from the Jews, nor to be partakers of their impiety.” APOSTOLIC CANONS — CANON 70: “ If any bishop, presbyter, or deacon, or any one of the list of clergy, keeps fast or festival with the Jews, or receives from them any of the gifts of their feasts, as unleavened bread, any such things, let him be deposed. If he be a layman, let him be excommunicated.” HEBREWS 8:13: “Now in saying a new, he hath made the former old. And that which decayeth and groweth old, is near its end.” COLOSSIANS 2:16–17: “Let no man therefore judge you in meat or in drink, or in respect of a festival day, or of the new moon, or of the sabbaths, Which are a shadow of things to come, but the body is of Christ.” GALATIANS 5:2–4: “Behold, I Paul tell you, that if you be circumcised, Christ shall profit you nothing.” ST. IGNATIUS OF ANTIOCH: “It is absurd to profess Christ Jesus, and to Judaize.” St. Augustine — Letter 82, §18: "Whoever observes these [Jewish] ceremonies, whether he was originally Jew or Gentile, is on his way to the pit of perdition, not only if he is sincerely observing them, but also if he is observing them with dissimulation." St. Jerome — Letter 112, §14: "I, on the contrary, shall maintain, and, though the world were to protest against my view, I may boldly declare that the Jewish ceremonies are to Christians both hurtful and fatal" St. John Chrysostom — Against the Judaizers, Homily I, §5: "If a man sees you that have knowledge come into the synagogue and participate in the festival of the Trumpets, shall not his conscience, being weak, be emboldened to admire what the Jews do?" St. John Chrysostom — Against the Judaizers, Homily III: "Christ did keep the Pasch with them. Yet he did not do so with the idea that we should keep the Pasch with them." St. John Chrysostom — Against the Judaizers, Homily IV, §3: "Do you share with us in the mysteries, do you worship Christ as a Christian, do you ask him for blessings, and do you then celebrate the festival with his foes?" ST. THOMAS AQUINAS — SUMMA THEOLOGIAE, I–II, QUESTION 103, ARTICLES 3 AND 4: “ Article 3. Whether the ceremonies of the Old Law ceased at the coming of Christ? Objection 1. It would seem that the ceremonies of the Old Law did not cease at the coming of Christ. For it is written (Baruch 4:1): "This is the book of the commandments of God, and the law that is for ever." But the legal ceremonies were part of the Law. Therefore the legal ceremonies were to last for ever. Objection 2. Further, the offering made by a leper after being cleansed was a ceremony of the Law. But the Gospel commands the leper, who has been cleansed, to make this offering (Matthew 8:4). Therefore the ceremonies of the Old Law did not cease at Christ's coming. Objection 3. Further, as long as the cause remains, the effect remains. But the ceremonies of the Old Law had certain reasonable causes, inasmuch as they were ordained to the worship of God, besides the fact that they were intended to be figures of Christ. Therefore the ceremonies of the Old Law should not have ceased. Objection 4. Further, circumcision was instituted as a sign of Abraham's faith: the observance of the sabbath, to recall the blessing of creation: and other solemnities, in memory of other Divine favors, as state above (I-II:102:4 ad 10; Article 5, Reply to Objection 1). But Abraham's faith is ever to be imitated even by us: and the blessing of creation and other Divine favors should never be forgotten. Therefore at least circumcision and the other legal solemnities should not have ceased. On the contrary, The Apostle says (Colossians 2:16-17): "Let no man . . . judge you in meat or in drink, or in respect of a festival day, or of the new moon, or of the sabbaths, which are a shadow of things to come": and (Hebrews 8:13): "In saying a new (testament), he hath made the former old: and that which decayeth and groweth old, is near its end." I answer that, All the ceremonial precepts of the Old Law were ordained to the worship of God as stated above (I-II:101:1-2). Now external worship should be in proportion to the internal worship, which consists in faith, hope and charity. Consequently exterior worship had to be subject to variations according to the variations in the internal worship, in which a threefold state may be distinguished. One state was in respect of faith and hope, both in heavenly goods, and in the means of obtaining them—in both of these considered as things to come. Such was the state of faith and hope in the Old Law. Another state of interior worship is that in which we have faith and hope in heavenly goods as things to come; but in the means of obtaining heavenly goods, as in things present or past. Such is the state of the New Law. The third state is that in which both are possessed as present; wherein nothing is believed in as lacking, nothing hoped for as being yet to come. Such is the state of the Blessed. In this state of the Blessed, then, nothing in regard to worship of God will be figurative; there will be naught but "thanksgiving and voice of praise" (Isaiah 51:3). Hence it is written concerning the city of the Blessed (Apocalypse 21:22): "I saw no temple therein: for the Lord God Almighty is the temple thereof, and the Lamb." Proportionately, therefore, the ceremonies of the first-mentioned state which foreshadowed the second and third states, had need to cease at the advent of the second state; and other ceremonies had to be introduced which would be in keeping with the state of divine worship for that particular time, wherein heavenly goods are a thing of the future, but the Divine favors whereby we obtain the heavenly boons are a thing of the present. Reply to Objection 1. The Old Law is said to be "for ever" simply and absolutely, as regards its moral precepts; but as regards the ceremonial precepts it lasts for even in respect of the reality which those ceremonies foreshadowed. Reply to Objection 2. The mystery of the redemption of the human race was fulfilled in Christ's Passion: hence Our Lord said then: "It is consummated" (John 19:30). Consequently the prescriptions of the Law must have ceased then altogether through their reality being fulfilled. As a sign of this, we read that at the Passion of Christ "the veil of the temple was rent" (Matthew 27:51). Hence, before Christ's Passion, while Christ was preaching and working miracles, the Law and the Gospel were concurrent, since the mystery of Christ had already begun, but was not as yet consummated. And for this reason Our Lord, before His Passion, commanded the leper to observe the legal ceremonies. Reply to Objection 3. The literal reasons already given (I-II:102) for the ceremonies refer to the divine worship, which was founded on faith in that which was to come. Hence, at the advent of Him Who was to come, both that worship ceased, and all the reasons referring thereto. Reply to Objection 4. The faith of Abraham was commended in that he believed in God's promise concerning his seed to come, in which all nations were to blessed. Wherefore, as long as this seed was yet to come, it was necessary to make profession of Abraham's faith by means of circumcision. But now that it is consummated, the same thing needs to be declared by means of another sign, viz. Baptism, which, in this respect, took the place of circumcision, according to the saying of the Apostle (Colossians 2:11-12): "You are circumcised with circumcision not made by hand, in despoiling of the body of the flesh, but in the circumcision of Christ, buried with Him in Baptism." As to the sabbath, which was a sign recalling the first creation, its place is taken by the "Lord's Day," which recalls the beginning of the new creature in the Resurrection of Christ. In like manner other solemnities of the Old Law are supplanted by new solemnities: because the blessings vouchsafed to that people, foreshadowed the favors granted us by Christ. Hence the feast of the Passover gave place to the feast of Christ's Passion and Resurrection: the feast of Pentecost when the Old Law was given, to the feast of Pentecost on which was given the Law of the living spirit: the feast of the New Moon, to Lady Day, when appeared the first rays of the sun, i.e. Christ, by the fulness of grace: the feast of Trumpets, to the feasts of the Apostles: the feast of Expiation, to the feasts of Martyrs and Confessors: the feast of Tabernacles, to the feast of the Church Dedication: the feast of the Assembly and Collection, to feast of the Angels, or else to the feast of All Hallows. Article 4. Whether since Christ's Passion the legal ceremonies can be observed without committing mortal sin? Objection 1. It would seem that since Christ's Passion the legal ceremonies can be observed without committing mortal sin. For we must not believe that the apostles committed mortal sin after receiving the Holy Ghost: since by His fulness they were "endued with power from on high" (Luke 24:49). But the apostles observed the legal ceremonies after the coming of the Holy Ghost: for it is stated (Acts 16:3) that Paul circumcised Timothy: and (Acts 21:26) that Paul, at the advice of James, "took the men, and . . . being purified with them, entered into the temple, giving notice of the accomplishment of the days of purification, until an oblation should be offered for every one of them." Therefore the legal ceremonies can be observed since the Passion of Christ without mortal sin. Objection 2. Further, one of the legal ceremonies consisted in shunning the fellowship of Gentiles. But the first Pastor of the Church complied with this observance; for it is stated (Galatians 2:12) that, "when" certain men "had come" to Antioch, Peter "withdrew and separated himself" from the Gentiles. Therefore the legal ceremonies can be observed since Christ's Passion without committing mortal sin. Objection 3. Further, the commands of the apostles did not lead men into sin. But it was commanded by apostolic decree that the Gentiles should observe certain ceremonies of the Law: for it is written (Acts 15:28-29): "It hath seemed good to the Holy Ghost and to us, to lay no further burden upon you than these necessary things: that you abstain from things sacrificed to idols, and from blood, and from things strangled, and from fornication." Therefore the legal ceremonies can be observed since Christ's Passion without committing mortal sin. On the contrary, The Apostle says (Galatians 5:2): "If you be circumcised, Christ shall profit you nothing." But nothing save mortal sin hinders us from receiving Christ's fruit. Therefore since Christ's Passion it is a mortal sin to be circumcised, or to observe the other legal ceremonies. I answer that, All ceremonies are professions of faith, in which the interior worship of God consists. Now man can make profession of his inward faith, by deeds as well as by words: and in either profession, if he make a false declaration, he sins mortally. Now, though our faith in Christ is the same as that of the fathers of old; yet, since they came before Christ, whereas we come after Him, the same faith is expressed in different words, by us and by them. For by them was it said: "Behold a virgin shall conceive and bear a son," where the verbs are in the future tense: whereas we express the same by means of verbs in the past tense, and say that she "conceived and bore." In like manner the ceremonies of the Old Law betokened Christ as having yet to be born and to suffer: whereas our sacraments signify Him as already born and having suffered. Consequently, just as it would be a mortal sin now for anyone, in making a profession of faith, to say that Christ is yet to be born, which the fathers of old said devoutly and truthfully; so too it would be a mortal sin now to observe those ceremonies which the fathers of old fulfilled with devotion and fidelity. Such is the teaching Augustine (Contra Faust. xix, 16), who says: "It is no longer promised that He shall be born, shall suffer and rise again, truths of which their sacraments were a kind of image: but it is declared that He is already born, has suffered and risen again; of which our sacraments, in which Christians share, are the actual representation." Reply to Objection 1. On this point there seems to have been a difference of opinion between Jerome and Augustine. For Jerome (Super Galat. ii, 11, seqq.) distinguished two periods of time. One was the time previous to Christ's Passion, during which the legal ceremonies were neither dead, since they were obligatory, and did expiate in their own fashion; nor deadly, because it was not sinful to observe them. But immediately after Christ's Passion they began to be not only dead, so as no longer to be either effectual or binding; but also deadly, so that whoever observed them was guilty of mortal sin. Hence he maintained that after the Passion the apostles never observed the legal ceremonies in real earnest; but only by a kind of pious pretense, lest, to wit, they should scandalize the Jews and hinder their conversion. This pretense, however, is to be understood, not as though they did not in reality perform those actions, but in the sense that they performed them without the mind to observe the ceremonies of the Law: thus a man might cut away his foreskin for health's sake, not with the intention of observing legal circumcision. But since it seems unbecoming that the apostles, in order to avoid scandal, should have hidden things pertaining to the truth of life and doctrine, and that they should have made use of pretense, in things pertaining to the salvation of the faithful; therefore Augustine (Epist. lxxxii) more fittingly distinguished three periods of time. One was the time that preceded the Passion of Christ, during which the legal ceremonies were neither deadly nor dead: another period was after the publication of the Gospel, during which the legal ceremonies are both dead and deadly. The third is a middle period, viz. from the Passion of Christ until the publication of the Gospel, during which the legal ceremonies were dead indeed, because they had neither effect nor binding force; but were not deadly, because it was lawful for the Jewish converts to Christianity to observe them, provided they did not put their trust in them so as to hold them to be necessary unto salvation, as though faith in Christ could not justify without the legal observances. On the other hand, there was no reason why those who were converted from heathendom to Christianity should observe them. Hence Paul circumcised Timothy, who was born of a Jewish mother; but was unwilling to circumcise Titus, who was of heathen nationality. The reason why the Holy Ghost did not wish the converted Jews to be debarred at once from observing the legal ceremonies, while converted heathens were forbidden to observe the rites of heathendom, was in order to show that there is a difference between these rites. For heathenish ceremonial was rejected as absolutely unlawful, and as prohibited by God for all time; whereas the legal ceremonial ceased as being fulfilled through Christ's Passion, being instituted by God as a figure of Christ. Reply to Objection 2. According to Jerome, Peter withdrew himself from the Gentiles by pretense, in order to avoid giving scandal to the Jews, of whom he was the Apostle. Hence he did not sin at all in acting thus. On the other hand, Paul in like manner made a pretense of blaming him, in order to avoid scandalizing the Gentiles, whose Apostle he was. But Augustine disapproves of this solution: because in the canonical Scripture (viz. Galatians 2:11), wherein we must not hold anything to be false, Paul says that Peter "was to be blamed." Consequently it is true that Peter was at fault: and Paul blamed him in very truth and not with pretense. Peter, however, did not sin, by observing the legal ceremonial for the time being; because this was lawful for him who was a converted Jew. But he did sin by excessive minuteness in the observance of the legal rites lest he should scandalize the Jews, the result being that he gave scandal to the Gentiles. Reply to Objection 3. Some have held that this prohibition of the apostles is not to be taken literally, but spiritually: namely, that the prohibition of blood signifies the prohibition of murder; the prohibition of things strangled, that of violence and rapine; the prohibition of things offered to idols, that of idolatry; while fornication is forbidden as being evil in itself: which opinion they gathered from certain glosses, which expound these prohibitions in a mystical sense. Since, however, murder and rapine were held to be unlawful even by the Gentiles, there would have been no need to give this special commandment to those who were converted to Christ from heathendom. Hence others maintain that those foods were forbidden literally, not to prevent the observance of legal ceremonies, but in order to prevent gluttony. Thus Jerome says on Ezekiel 44:31 ("The priest shall not eat of anything that is dead"): "He condemns those priests who from gluttony did not keep these precepts." But since certain foods are more delicate than these and more conducive to gluttony, there seems no reason why these should have been forbidden more than the others. We must therefore follow the third opinion, and hold that these foods were forbidden literally, not with the purpose of enforcing compliance with the legal ceremonies, but in order to further the union of Gentiles and Jews living side by side. Because blood and things strangled were loathsome to the Jews by ancient custom; while the Jews might have suspected the Gentiles of relapse into idolatry if the latter had partaken of things offered to idols. Hence these things were prohibited for the time being, during which the Gentiles and Jews were to become united together. But as time went on, with the lapse of the cause, the effect lapsed also, when the truth of the Gospel teaching was divulged, wherein Our Lord taught that "not that which entereth into the mouth defileth a man" (Matthew 15:11); and that "nothing is to be rejected that is received with thanksgiving" (1 Timothy 4:4). With regard to fornication a special prohibition was made, because the Gentiles did not hold it to be sinful.” If you still don’t believe, well, at least I tried. 🤷‍♂️