Here’s the thing. I’m atheist. I don’t care what you are. You could be Catholic, Muslim, Buddhist or Laveyan Satanist. No skin off my nose.
What I’m interested in is what you believe and how that impacts your actions. If being a good Catholic means you support social equality, economic justice and otherwise being a good person: good for you!
Your motivation for being a good person does not concern me. But if you hide your awfulness behind the pretense of religion then yes, I can and will challenge your beliefs because I want to change your behaviour. Maybe I won’t succeed. But honest debate can and does convert people; maybe not immediately, but often weeks or months or years down the line it can be the pebble that started an avalanche.
I have had a huge number of people I know who became atheists specifically because they were angered enough in a debate with atheists to try and prove them wrong using evidence… only to not find it. Sometimes, by forcing a person to defend his beliefs, you can change those beliefs because a person may not be even aware what he is accepting without thinking.
This is why I want to know what the other person in the debate believes. My goal is to not only learn but help the other person learn. To serve as a excuse for someone to pull up all those dusty old unexamined prejudices and examine them again.
And if the other person makes me do the same? All the better.