Q: An infant is baptized. The parents live and bring this child up learning the Word. This child withdraws from grace later in life when they are on their own. As an adult they come back to church and seek the grace of God. This person then wants to be baptized again in their quest for a new life. Would you baptize again since we believe in one baptism?
A: This is a fantastic question that gets to the heart of the Biblical teaching concerning Baptism and faith and the relationship between the two. Is baptism valid because of our faith, or is it valid regardless if we believe it or not? If baptism is built on the foundation of our faith, then it makes sense to re-baptize if someone loses their faith and then later believes. On the other hand, if the validity of Baptism is based on something else, something apart from us, something like the sure promises of God, then there is no need to re-baptize. Baptism stands even when we have fallen.
It is indeed the case that Baptism is valid not because of our faith, but because of the word and promise of God. Baptism is loaded with the promises of God: the promise of salvation [St Mark 16:16; 1 Peter 3:21], the promise of forgiveness [Acts 2:38; 22:16], the promise the we are born-again and given God's own name [St John 3:5; Titus 3:5; St Matthew 28:18-20] and God washes us clean [Ephesians 5:25-26]. All of these things are true, even if we don't believe it.
Imagine a boy whose relative dies and leaves them a huge inheritance. “You're a millionaire,” everyone says, but the boy doesn't believe it. He doesn't trust the promises. Does his lack of faith make the inheritance invalid? No, the money is still there, but the boy doesn't enjoy the benefits of it. Say later in life the boy is convinced that he really did inherit all that money, does he need to be given the inheritance again? No, he simply claims what is already his. So it is with baptism, God is really giving us the gifts of life, salvation and the forgiveness of all of our sins, naming us as His children and inheritors of His heavenly kingdom. If we walk away from those gifts and promises they are still there; they always remain as solid and sure as all of God's words.
This question, by the way, arose during the reformation. The so-called “Anabaptists” (which means “another baptism”) taught that the only valid baptism was of someone who demonstrated their faith in acts of obedience. Luther responds to them in any number of places, but here is a bit of his thinking from the Large Catechism:
Further, we are not primarily concerned whether the baptized person believes or not, for in the latter case Baptism does not become invalid. Everything depends on the Word and commandment of God. This, perhaps, is a rather subtle point, but it is based upon what I have already said, that Baptism is simply water and God's Word in and with each other; that is, when the Word accompanies the water, Baptism is valid, even though faith be lacking. For my faith does not constitute Baptism but receives it. Baptism does not become invalid even if it is wrongly received or used, for it is bound not to our faith but to the Word.
[Martin Luther, The Large Catechism, IV.52-53. Italics added.]
Our baptism is sure and certain, for it is the giving out of the rock-solid promises of God. Even when our faith wavers and staggers, God's faithfulness is sure. His faithfulness is our hope and comfort and peace, and so we rejoice, knowing that we are baptized. God has given to us all the treasures of heaven.