Outline - Concerning Rebaptism: A Letter to Two Pastors (PDF)
(Concerning Rebaptism can be found in Works of Luther #40)
Luther chides his readers, if they are papists, for complaining about the "re-baptizers" when, in a sense, every papist is a sort of Anabaptist! He argues that these people do not deserve to be so lamentably murdered, burned and tormented to death.
He notes that, there remains much that is Christian and good under the papacy (Scripture, Baptism, the Supper, the Keys, the ministry, the content of the Catechism, etc.), so if the Anabaptists are justifying everything by "not doing what the Antichrist does", they are simply wrong.
It's the same with, "We do not want to believe what men say." Luther says, "This is lose everything even in your own history: mother, father, Baptism - you believe these things took place because men have told you that they did." He illustrates how, in some cases, we believe because the Word of God tells us how things are. One of the things we learn there is that Baptism is a work of God - not a "work of men."
He tackles their use of Mk. 16:16, as they use it to argue that no one should be baptized until after he believes. Not only is this not Scriptural; no one can be absolutely sure that he believes. They claim that "...children cannot believe," but how do they know that? (Luther here marshals many passages against them.)
Then he tackles their charge that "...it was not done right the first time." But even if that were true, one would not have to "do it again!" (he explains why, even compares it to a girl forced to marry a man she did not love; if she came to have affection for him, would they ask to be remarried?)
The truth of the matter is that the Anabaptists, though they talk about "faith," are really reintroducing a doctrine of works here - something that pleases the devil. Luther then expounds the many good reasons to baptize children and to hold that they are believers in Christ.