Luther on Baptism Against the Anabaptists

That request, however, should not be granted!   Instead, insists Luther, the one making this request should be told that even if he were thus to be ‘baptized’ a second time — Satan might well soon trouble him again, as to whether he then too really had faith.   Then he would have to be ‘baptized’ yet again — a third time — and so on, ad infinitum, for just as long as any such doubts kept recurring.

"For it often happens that one who thinks that he has faith," explains Luther, "has none whatever — and that one who thinks that he has no faith but only doubts, actually believes. We are not told ‘he who knows he believes’ nor ‘he who thinks he is sure he believes’ [shall be saved]…, but ‘he that believes [and is baptized] shall be saved’ [Mark 16:16]….