9 But after the announcement of the mystery of Christ's nature, and our assumption, that is, the fullness of Godhead abiding in Christ, and ourselves made full in Him by His birth as man, the Apostle continues the dispensation of human salvation in the words, In whom you were also circumcised with a circumcision not made with hands, in the stripping off of the body of the flesh, but with the circumcision of Christ, having been buried with Him in baptism, wherein you were also raised with Him through faith in the working of God, who raised Him from the dead. We are circumcised not with a fleshly circumcision but with the circumcision of Christ, that is, we are born again into a new man; for, being buried with Him in His baptism, we must die to the old man, because the regeneration of baptism has the force of resurrection.
The circumcision of Christ does not mean the putting off of foreskins, but to die entirely with Him, and by that death to live henceforth entirely to Him. For we rise again in Him through faith in God, Who raised Him from the dead; wherefore we must believe in God, by Whose Working Christ was raised from the dead, for our faith rises again in and with Christ.
Source: On the Trinity (New Advent)