How does a Lutheran become a Baptist?

Q: How does a Lutheran become a Baptist? 

A: By reading Martin Luther!

Baptism...means to plunge something completely into the water, so that the water covers it... This usage is also demanded by the significance of the baptism itself. For baptism, as we shall hear, signifies that the old man and the sinful birth of flesh and blood are to be wholly drowned by the grace of God. We should therefore do justice to its meaning and make baptism a true and complete sign of the thing it signifies.

Martin Luther, Luther’s Works, vol.35, p. 29, Fortress Press, 1960

(HT: ES)