Baptism is not just plain water, but it is water contained within God's
command and united with God's Word.

Where in the Word of God is this?

Where our Lord Christ spoke in the last chapter of Matthew (Matthew
28:19):
"Go into all the world, teaching all heathen nations, and baptizing
them in the name of the Father, the Son and of the Holy Spirit."

What does Baptism give? What good is it?

It gives us the forgiveness of sins, redeems us from death and the
Devil, and gives eternal salvation to all who believe this, just as
God's words and promises declare.

What are these words and promises of God?

Our Lord Christ spoke one of them in the last chapter of Mark (Mark
16:16):
"Whoever believes and is baptized will be saved; but whoever does not
believe will be damned."

How can water do such great things?

Water doesn't make these things happen, of course. It is God's Word,
which is with and in the water. Because, without God's Word, the water
is plain water and not baptism. But with God's Word it is a Baptism, a
grace-filled water of life, a bath of new birth in the Holy Spirit, as
St. Paul said to Titus in the third chapter (Titus 3:5-8):