The only way you can truly not judge somebody by how they look is to be blind. Everyone does it, it's just to what degree you judge, and to what degree you act upon said judgement.
Now, I agree that looking down upon somebody based on these judgements is a really a** thing to do. For instance:
Cruciarius
i agree. i can't have a good conversation with christians either.
This person will deem you being of lower than average intelligence if you take part in a certain religion. Pretty uncool of him, eh?
However, there are much worse things you can do. Murder, for instance. Or perhaps rape.