I say this in order that we may not adopt the opinion that long prevailed among us, that our Baptism is past and can never be used after we again have fallen into sin. The reason for a notion of this sort is that one looks upon Baptism in its aspect of a one-time action. Indeed, the idea goes back to ST. Jerome, who wrote, "Repentance is the second plank on which we must swim ashore after the sinking of the ship" in which we embarked and sailed forth when we entered the Christian church at Baptism.
This view deprives us of the use of Baptism, so that it can no longer benefit us. Jerome's statement is not correct, for the ship, Baptism, cannot be shipwrecked because, as we said, it is God's ordinance and not something of ours. It can happen, however, that we slip and fall out of the ship. But when this happens and someone does fall out, he should at once swim for the ship again and cling to it until he can climb back on deck and sail forward as he had earlier begun to do.
Thus we see how splendid a thing Baptism is. It snatches us out of the devil's jaws, makes God our own, defeats and puts away sin, daily strengthens the new man is us, keeps functioning, and remains with us until we leave our present troubles to enter glory everlasting.
Everyone should therefore look upon his Baptism as his everyday wear, to be worn continually. He should at all times be found in faith and surrounded by its fruits. He should every day suppress the old man and grow toward maturity in the new man. For if we want to be Christians, we must carry on in the work that makes us Christians.
But if someone falls away from it, let him come back to it again. For just as Christ, the throne of divine mercy, does not withdraw from us nor prevent us from coming back again to Him, though we sin, so also all His treasures and gifts remain. As we once obtained forgiveness of sins in Baptism, so that forgiveness continues for us as long as we live, that is to say, for as long as we have the old Adam hanging about our necks.
Martin Luther, "The Large Catechism"
from "For All the Saints" pages 217-218