The Church was redeemed at the price of Christ's blood. Jew or Greek, it makes no difference; but if he has believed, he must circumcise himself from his sins [in baptism (Col. 2:11-12)] so that he can be saved . . . for no one ascends into the kingdom of heaven except through the sacrament of baptism . . . "Unless a man be born again of water and the Holy Spirit, he cannot enter the kingdom of God"
Ambrose... the Apostle Paul says that we are sealed in the Spirit (cf. Eph. 1:13); since we have in the Son the image of the Father, and in the Spirit the seal of the Son. Let us, then, sealed by this Trinity, take more diligent heed, lest either levity of character or the deceit of any unfaithfulness unseal the pledge which we have received in our hearts.
Ambrose