We ask for what reason our Lord was unwilling to state the time of His coming (cf. Mk. 13:31-32). If we ask it, we shall not find it is owing to ignorance, but to wisdom. For it was not to our advantage to know; in order that we being ignorant of the actual moments of judgment to come, might ever be as it were on guard, and set on the watch-tower of virtue, and so avoid the habits of sin; lest the day of the Lord should come upon us in the midst of our wickedness.
Ambrose... these three witnesses are one, as John said: 'The water, the blood, and the Spirit' (I Jn. 5:8). One in the mystery, not in nature. The water, then, is a witness of burial, the blood is a witness of death, the Spirit is a witness of life. If, then, there be any grace in the water, it is not from the nature of water, but from the presence of the Holy Spirit.
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.
AmbroseThe 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