Objection 1. It would seem that the ceremonies of the Old Law had the power of justification at the time of the Law. Because expiation from sin and consecration pertains to justification. But it is written (Exodus 39:21) that the priests and their apparel were consecrated by the sprinkling of blood and the anointing of oil; and (Leviticus 16:16) that, by sprinkling the blood of the calf, the priest expiated "the sanctuary from the uncleanness of the children of Israel, and from their transgressions and... their sins." Therefore the ceremonies of the Old Law had the power of justification.
Objection 2. Further, that by which man pleases God pertains to justification, according to Psalm 10:8: "The Lord is just and hath loved justice." But some pleased God by means of ceremonies, according to Leviticus 10:19: "How could I... please the Lord in the ceremonies, having a sorrowful heart?" Therefore the ceremonies of the Old Law had the power of justification.
Objection 3. Further, things relating to the divine worship regard the soul rather than the body, according to Psalm 18:8: "The Law of the Lord is unspotted, converting souls." But the leper was cleansed by means of the ceremonies of the Old Law, as stated in Leviticus 14. Much more therefore could the ceremonies of the Old Law cleanse the soul by justifying it.
On the contrary, The Apostle says (Galatians 2) [The first words of the quotation are from 3:21: St. Thomas probably quoting from memory, substituted them for 2:21, which runs thus: 'If justice be by the Law, then Christ died in vain.']: "If there had been a law given which could justify [Vulgate: 'give life'], Christ died in vain," i.e. without cause. But this is inadmissible. Therefore the ceremonies of the Old Law did not confer justice.
I answer that, As stated above (102, 5, ad 4), a twofold uncleanness was distinguished in the Old Law. One was spiritual and is the uncleanness of sin. The other was corporal, which rendered a man unfit for divine worship; thus a leper, or anyone that touched carrion, was said to be unclean: and thus uncleanness was nothing but a kind of irregularity. From this uncleanness, then, the ceremonies of the Old Law had the power to cleanse: because they were ordered by the Law to be employed as remedies for the removal of the aforesaid uncleannesses which were contracted in consequence of the prescription of the Law. Hence the Apostle says (Hebrews 9:13) that "the blood of goats and of oxen, and the ashes of a heifer, being sprinkled, sanctify such as are defiled, to the cleansing of the flesh." And just as this uncleanness which was washed away by such like ceremonies, affected the flesh rather than the soul, so also the ceremonies themselves are called by the Apostle shortly before (Hebrews 9:10) justices of the flesh: "justices of the flesh," says he, "being laid on them until the time of correction."
On the other hand, they had no power of cleansing from uncleanness of the soul, i.e. from the uncleanness of sin. The reason of this was that at no time could there be expiation from sin, except through Christ, "Who taketh away the sins [Vulgate: 'sin'] of the world" (John 1:29). And since the mystery of Christ's Incarnation and Passion had not yet really taken place, those ceremonies of the Old Law could not really contain in themselves a power flowing from Christ already incarnate and crucified, such as the sacraments of the New Law contain. Consequently they could not cleanse from sin: thus the Apostle says (Hebrews 10:4) that "it is impossible that with the blood of oxen and goats sin should be taken away"; and for this reason he calls them (Galatians 4:9) "weak and needy elements": weak indeed, because they cannot take away sin; but this weakness results from their being needy, i.e. from the fact that they do not contain grace within themselves.
However, it was possible at the time of the Law, for the minds of the faithful, to be united by faith to Christ incarnate and crucified; so that they were justified by faith in Christ: of which faith the observance of these ceremonies was a sort of profession, inasmuch as they foreshadowed Christ. Hence in the Old Law certain sacrifices were offered up for sins, not as though the sacrifices themselves washed sins away, but because they were professions of faith which cleansed from sin. In fact, the Law itself implies this in the terms employed: for it is written (Leviticus 4:26; 5:16) that in offering the sacrifice for sin "the priest shall pray for him... and it shall be forgiven him," as though the sin were forgiven, not in virtue of the sacrifices, but through the faith and devotion of those who offered them. It must be observed, however, that the very fact that the ceremonies of the Old Law washed away uncleanness of the body, was a figure of that expiation from sins which was effected by Christ.
It is therefore evident that under the state of the Old Law the ceremonies had no power of justification.
Reply to Objection 1. That sanctification of priests and their sons, and of their apparel or of anything else belonging to them, by sprinkling them with blood, had no other effect but to appoint them to the divine worship, and to remove impediments from them, "to the cleansing of the flesh," as the Apostle states (Hebrews 9:13) in token of that sanctification whereby "Jesus" sanctified "the people by His own blood" (Hebrews 13:12). Moreover, the expiation must be understood as referring to the removal of these bodily uncleannesses, not to the forgiveness of sin. Hence even the sanctuary which could not be the subject of sin is stated to be expiated.
Reply to Objection 2. The priests pleased God in the ceremonies by their obedience and devotion, and by their faith in the reality foreshadowed; not by reason of the things considered in themselves.
Reply to Objection 3. Those ceremonies which were prescribed in the cleansing of a leper, were not ordained for the purpose of taking away the defilement of leprosy. This is clear from the fact that these ceremonies were not applied to a man until he was already healed: hence it is written (Leviticus 14:3-4) that the priest, "going out of the camp, when he shall find that the leprosy is cleansed, shall command him that is to be purified to offer," etc.; whence it is evident that the priest was appointed the judge of leprosy, not before, but after cleansing. But these ceremonies were employed for the purpose of taking away the uncleanness of irregularity. They do say, however, that if a priest were to err in his judgment, the leper would be cleansed miraculously by the power of God, but not in virtue of the sacrifice. Thus also it was by miracle that the thigh of the adulterous woman rotted, when she had drunk the water "on which" the priest had "heaped curses," as stated in Numbers 5:19-27.
Source: Summa Theologica (New Advent)