Objection 1. It seems that the sacraments of the Old Law caused grace. For, as stated above (5, ad 2) the sacraments of the New Law derive their efficacy from faith in Christ's Passion. But there was faith in Christ's Passion under the Old Law, as well as under the New, since we have "the same spirit of faith" (2 Corinthians 4:13). Therefore just as the sacraments of the New Law confer grace, so did the sacraments of the Old Law.
Objection 2. Further, there is no sanctification save by grace. But men were sanctified by the sacraments of the Old Law: for it is written (Leviticus 8:31): "And when he," i.e. Moses, "had sanctified them," i.e. Aaron and his sons, "in their vestments," etc. Therefore it seems that the sacraments of the Old Law conferred grace.
Objection 3. Further, Bede says in a homily on the Circumcision: "Under the Law circumcision provided the same health-giving balm against the wound of original sin, as baptism in the time of revealed grace." But Baptism confers grace now. Therefore circumcision conferred grace; and in like manner, the other sacraments of the Law; for just as Baptism is the door of the sacraments of the New Law, so was circumcision the door of the sacraments of the Old Law: hence the Apostle says (Galatians 5:3): "I testify to every man circumcising himself, that he is a debtor to the whole law."
On the contrary, It is written (Galatians 4:9): "Turn you again to the weak and needy elements?" i.e. "to the Law," says the gloss, "which is called weak, because it does not justify perfectly." But grace justifies perfectly. Therefore the sacraments of the old Law did not confer grace.
I answer that, It cannot be said that the sacraments of the Old Law conferred sanctifying grace of themselves, i.e. by their own power: since thus Christ's Passion would not have been necessary, according to Galatians 2:21: "If justice be by the Law, then Christ died in vain."
But neither can it be said that they derived the power of conferring sanctifying grace from Christ's Passion. For as it was stated above (5), the power of Christ's Passion is united to us by faith and the sacraments, but in different ways; because the link that comes from faith is produced by an act of the soul; whereas the link that comes from the sacraments, is produced by making use of exterior things. Now nothing hinders that which is subsequent in point of time, from causing movement, even before it exists in reality, in so far as it pre-exists in an act of the soul: thus the end, which is subsequent in point of time, moves the agent in so far as it is apprehended and desired by him. On the other hand, what does not yet actually exist, does not cause movement if we consider the use of exterior things. Consequently, the efficient cause cannot in point of time come into existence after causing movement, as does the final cause. It is therefore clear that the sacraments of the New Law do reasonably derive the power of justification from Christ's Passion, which is the cause of man's righteousness; whereas the sacraments of the Old Law did not.
Nevertheless the Fathers of old were justified by faith in Christ's Passion, just as we are. And the sacraments of the old Law were a kind of protestation of that faith, inasmuch as they signified Christ's Passion and its effects. It is therefore manifest that the sacraments of the Old Law were not endowed with any power by which they conduced to the bestowal of justifying grace: and they merely signified faith by which men were justified.
Reply to Objection 1. The Fathers of old had faith in the future Passion of Christ, which, inasmuch as it was apprehended by the mind, was able to justify them. But we have faith in the past Passion of Christ, which is able to justify, also by the real use of sacramental things as stated above.
Reply to Objection 2. That sanctification was but a figure: for they were said to be sanctified forasmuch as they gave themselves up to the Divine worship according to the rite of the Old Law, which was wholly ordained to the foreshadowing of Christ's Passion.
Reply to Objection 3. There have been many opinions about Circumcision. For, according to some, Circumcision conferred no grace, but only remitted sin. But this is impossible; because man is not justified from sin save by grace, according to Romans 3:24: "Being justified freely by His grace."
Wherefore others said that by Circumcision grace is conferred, as to the privative effects of sin, but not as to its positive effects. But this also appears to be false, because by Circumcision, children received the faculty of obtaining glory, which is the ultimate positive effect of grace. Moreover, as regards the order of the formal cause, positive effects are naturally prior to privative effects, though according to the order of the material cause, the reverse is the case: for a form does not exclude privation save by informing the subject.
Hence others say that Circumcision conferred grace also as regards a certain positive effect, i.e. by making man worthy of eternal life, but not so as to repress concupiscence which makes man prone to sin. And so at one time it seemed to me. But if the matter be considered carefully, this too appears to be untrue; because the very least grace is sufficient to resist any degree of concupiscence, and to merit eternal life.
And therefore it seems better to say that Circumcision was a sign of justifying faith: wherefore the Apostle says (Romans 4:11) that Abraham "received the sign of Circumcision, a seal of the justice of faith." Consequently grace was conferred in Circumcision in so far as it was a sign of Christ's future Passion, as will be made clear further on (70, 4).
Source: Summa Theologica (New Advent)