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."
Thomas Aquinas, Summa Theologiae 3.37.1.