I really can't stand the thought of bringing my mom to tears.
No healthy, normal mother would be sad about her child being honest, using his intelligence and logic, and doing the right thing.
If she is brought to tears over this, it is because her chosen religion has misled her to believe falsehoods about nonbelievers.
Therefore: Not your fault.
Imagine if we were talking about a person who believes in Thor, doesn't believe in going to college, dating, having careers, and insists everyone skip medical care, holidays, and many other normal/healthy things to serve Thor. Are you supposed to go along with this just because she might cry if you don't?
to stop preaching a message that I don't agree with
Yes, and more. The message destroys and actually ends lives. I nearly bled to death many times during childhood because of it--no exaggeration. And think of all those who have committed suicide or done other deeply self-destructive things because of the teachings and shunning. You can't preach a message that could result in someone's death. It's not right. (You know this.)