This forum: https://www.reddit.com/r/Christianity/
I've been warned a few times, but, never banned. That forum isn't about debate, but, if the original post asks a question, or a Christian raises a point to you, the moderators are pretty cool about allowing the non-believer to respond strongly.
The nice thing there is having discussions with believers that aren't just the brave few who venture to the religious debate forum. Those forums are just people talking at each other and getting nowhere.
I don't make posts on there saying, "This is all BS" but, I do respond to posts on there, pointing out gigantic holes in the Christian doctrine and not being overly nice about it... and so far, I'm still there.
Also, there are so many depressed Christians who are considering suicide or need help, and are confused as to why God doesn't answer, and feel unworthy. For those people, I actually try to help, and I don't generally tell them that their faith is a lie. I'll say things like, "If you believe, believe that God gave you logic and critical thinking for just this reason, and you've got to make strides to make these changes in your life with those gifts."
Or I'll empathize with them that I used to be a believer, and had the same doubts, and they're not alone, and that if God was real, he'd probably be more intelligent and empathetic than us, and he'd understand why I don't believe.