When they try and push their religion, I'll start pushing back. Each to their own, normally, I'll leave them alone. My wife won't answer the door when they knock. She doesn't want to see me start a conversation with them.

When they try and quote the bible, then I'll start my questioning.

When they proselytize, then I go full atheist. Or in the case they are Christians, I can go full Christian extremist and show why he/she is a bad christian and will go to hell anyway...

But in general I never discuss in the matters of religion, i discuss in the matters of science (hard science, social sciences, history etc), if the religion contradicts or find a "cosmic nany explanation" for something, then I will challenge.

Normally not unless they make a public nuisance of themselves

Never...unless they challenge my disbelief.