I answer that, As stated above (Question [102], Article [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 (Heb. 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 (Heb. 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 [Vulg.: 'sin'] of the world" (Jn. 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 (Heb. 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 (Gal. 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 (Lev. 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.