Not a great argument, breh. You're right, Christians were horrible, too, but we shouldn't try and justify maltreatment by saying others did it, too.
The problem with religion is that it rarely helps truly eradicate racism unless it is taken up and transformed fundamentally by a group of people, as with Black Theology, Black Islam, etc. But in its original form, it rarely does, and is often predisposed towards the group of people with which it originates. On the occasion that it does eradicate some prejudice, it's usually contingent on the would-be victims of prejudice joining the religion before getting treated better.