Why was Christ circumcised? Saint Thomas Aquinas gives us seven reasons:

For several reasons Christ ought to have been circumcised.

1. First, in order to prove the reality of His human nature, in contradiction to the [heretical] Manicheans, who said that He had an imaginary body: and in contradiction to [the heretic] Apollinarius, who said that Christ’s body was consubstantial with His Godhead; and in contradiction to [the heretic] Valentine, who said that Christ brought His body from heaven. 
2. Secondly, in order to show His approval of circumcision, which God had instituted of old. 
3. 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. 
4. Fourthly, in order to take away from the Jews an excuse for not receiving Him, if He were uncircumcised. 
5. 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). 
6. 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.” 
7. 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.”

Remember also what the Apostle Paul says (Colossians 2:11): “In Christ you are circumcised with circumcision not made by hand in despoiling of the body of the flesh, but in the circumcision of Christ.” Hence, by baptism each of us has received the circumcision of Christ.

For more about the relationship between circumcision and baptism, please check out my book The Crucified Rabbi.