Great question.
I personally haven't gotten angry on a debate on religion, mainly because it's not a debate. The truth is not open to interpretation or opinion and cannot be debated.
I am a christian, and as such my foundation is in the Holy Bible. If I'm answering a question or in a discussion about religion, the Bible is the source of truth for me. I use scripture from the bible to answer questions and so if someone doesn't agree with my position, then it's not me that they aren't agreeing with, it's my God. I don't have a need to get emotional about it, because it's not about me personally.