We don't follow Martin Luther.
We follow Jesus.
Martin Luther failed to follow Jesus in many ways. We fail to follow Jesus in many ways. The nice part is, Jesus forgives us when we fail His standards.
The Bible tells us we will eventually let God down in a myriad of ways. So when we sin, even our sinning proves the Bible right.
However, Christians have in the Bible a moral code that allows us to declare some actions wrong: hence, when we say "Martin Luther was wrong to be anti-Semite," we have an Authority on which to base this statement. Unfortunately, atheists and those like them do not, because they have no such Authority to base their moral discernment on. Hence Christians can say "This fellow Christian was wrong by our standards," but atheists cannot say "this fellow atheist was wrong by our standards"--because they have no standards.
Know that when someone condemns Martin Luther, they are using a Christian standard to condemn him...what i personally find strange is that they then say this standard proves Christianity "wrong" or "bad." So a Christian standard that they agree with proves Christianity is bad? Hmm.
Just a little something to think about.