Objection 1. It would seem that Christ should not have been circumcised. For on the advent of the reality, the figure ceases. But circumcision was prescribed to Abraham as a sign of the covenant concerning his posterity, as may be seen from Genesis 17. Now this covenant was fulfilled in Christ's birth. Therefore circumcision should have ceased at once.
Objection 2. Further, "every action of Christ is a lesson to us" [Innoc. III, Serm. xxii de Temp.; wherefore it is written (John 3:15): "I have given you an example, that as I have done to you, so you do also." But we ought not to be circumcised; according to Galatians 5:2: "If you be circumcised, Christ shall profit you nothing." Therefore it seems that neither should Christ have been circumcised.
Objection 3. Further, circumcision was prescribed as a remedy of original sin. But Christ did not contract original sin, as stated above (14, 3; 15, 1). Therefore Christ should not have been circumcised.
On the contrary, It is written (Luke 2:21): "After eight days were accomplished, that the child should be circumcised."
I answer that, For several reasons Christ ought to have been circumcised. First, in order to prove the reality of His human nature, in contradiction to the Manicheans, who said that He had an imaginary body: and in contradiction to Apollinarius, who said that Christ's body was consubstantial with His Godhead; and in contradiction to Valentine, who said that Christ brought His body from heaven. Secondly, in order to show His approval of circumcision, which God had instituted of old. Thirdly, in order to prove that He was descended from Abraham, who had received the commandment of circumcision as a sign of his faith in Him. Fourthly, in order to take away from the Jews an excuse for not receiving Him, if He were uncircumcised. Fifthly, "in order by His example to exhort us to be obedient" [Bede, Hom. x in Evang.]. Wherefore He was circumcised on the eighth day according to the prescription of the Law (Leviticus 12:3). Sixthly, "that He who had come in the likeness of sinful flesh might not reject the remedy whereby sinful flesh was wont to be healed." Seventhly, that by taking on Himself the burden of the Law, He might set others free therefrom, according to Galatians 4:4-5: "God sent His Son... made under the Law, that He might redeem them who were under the Law."
Reply to Objection 1. Circumcision by the removal of the piece of skin in the member of generation, signified "the passing away of the old generation" [Athanasius, De Sabb. et Circumcis.]: from the decrepitude of which we are freed by Christ's Passion. Consequently this figure was not completely fulfilled in Christ's birth, but in His Passion, until which time the circumcision retained its virtue and status. Therefore it behooved Christ to be circumcised as a son of Abraham before His Passion.
Reply to Objection 2. Christ submitted to circumcision while it was yet of obligation. And thus His action in this should be imitated by us, in fulfilling those things which are of obligation in our own time. Because "there is a time and opportunity for every business" (Ecclesiastes 8:6).
Moreover, according to Origen (Hom. xiv in Luc.), "as we died when He died, and rose again when Christ rose from the dead, so were we circumcised spiritually through Christ: wherefore we need no carnal circumcision." And this is what the Apostle says (Colossians 2:11): "In whom," [i.e. Christ] "you are circumcised with circumcision not made by hand in despoiling of the body of the flesh, but in the circumcision of" our Lord Jesus "Christ."
Reply to Objection 3. As Christ voluntarily took upon Himself our death, which is the effect of sin, whereas He had no sin Himself, in order to deliver us from death, and to make us to die spiritually unto sin, so also He took upon Himself circumcision, which was a remedy against original sin, whereas He contracted no original sin, in order to deliver us from the yoke of the Law, and to accomplish a spiritual circumcision in us--in order, that is to say, that, by taking upon Himself the shadow, He might accomplish the reality.
Source: Summa Theologica (New Advent)