I answer that, Baptism is called the Sacrament of Faith; in so far, to wit, as in Baptism man makes a profession of faith, and by Baptism is aggregated to the congregation of the faithful. Now our faith is the same as that of the Fathers of old, according to the Apostle (2 Cor. 4:13): "Having the same spirit of faith . . . we . . . believe." But circumcision was a protestation of faith; wherefore by circumcision also men of old were aggregated to the body of the faithful. Consequently, it is manifest that circumcision was a preparation for Baptism and a figure thereof, forasmuch as "all things happened" to the Fathers of old "in figure" (1 Cor. 10:11); just as their faith regarded things to come.
Reply Obj. 1: Circumcision was like Baptism as to the spiritual effect of the latter. For just as circumcision removed a carnal pellicule, so Baptism despoils man of carnal behavior.
Reply Obj. 2: The protecting pillar of cloud and the crossing of the Red Sea were indeed figures of our Baptism, whereby we are born again of water, signified by the Red Sea; and of the Holy Ghost, signified by the pillar of cloud: yet man did not make, by means of these, a profession of faith, as by circumcision; so that these two things were figures but not sacraments. But circumcision was a sacrament, and a preparation for Baptism; although less clearly figurative of Baptism, as to externals, than the aforesaid. And for this reason the Apostle mentions them rather than circumcision.
Reply Obj. 3: John's baptism was a preparation for Christ's as to the act done: but circumcision, as to the profession of faith, which is required in Baptism, as stated above. _______________________
SECOND ARTICLE [III, Q. 70, Art. 2]
Whether Circumcision Was Instituted in a Fitting Manner?
Objection 1: It seems that circumcision was instituted in an unfitting manner. For as stated above (A. 1) a profession of faith was made in circumcision. But none could ever be delivered from the first man's sin, except by faith in Christ's Passion, according to Rom. 3:25: "Whom God hath proposed to be a propitiation, through faith in His blood." Therefore circumcision should have been instituted forthwith after the first man's sin, and not at the time of Abraham.
Obj. 2: Further, in circumcision man made profession of keeping the Old Law, just as in Baptism he makes profession of keeping the New Law; wherefore the Apostle says (Gal. 5:3): "I testify . . . to every man circumcising himself, that he is a debtor to do the whole Law." But the observance of the Law was not promulgated at the time of Abraham, but rather at the time of Moses. Therefore it was unfitting for circumcision to be instituted at the time of Abraham.
Obj. 3: Further, circumcision was a figure of, and a preparation for, Baptism. But Baptism is offered to all nations, according to Matt. 28:19: "Going . . . teach ye all nations, baptizing them." Therefore circumcision should have been instituted as binding, not the Jews only, but also all nations.
Obj. 4: Further, carnal circumcision should correspond to spiritual circumcision, as the shadow to the reality. But spiritual circumcision which is of Christ, regards indifferently both sexes, since "in Christ Jesus there is neither male nor female," as is written Col. 3 [*Gal. 3:28]. Therefore the institution of circumcision which concerns only males, was unfitting.