But actually, if you want to be more precise, Luther really didn't teach that we're saved by faith alone. I mean, when you look at everything that he said, because what he taught is that before the creation of mankind, we were all arbitrarily predestined by God for either salvation or for damnation. When I say arbitrarily, I mean it wasn't based on God's foreknowledge that he saw that we would have faith in his Son, so he determined then that we would be among the elect.
No, he made that decision based on no criteria. So if we have faith, it's because we were predestined to have faith. So it's not our faith that saves us; it's the predestination that saves or damns us.