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.
Source: Summa Theologica (New Advent)