I've seen them locked for two reasons. Either they were getting very contentious - and usually off topic; Or they were getting VERY long, and (again) off topic from the original question.
I'm sure that there are other reasons too, but I don't get into religious questions very often since they seem pointless. Either it is 'preaching to the choir', or it is ranting against everyone that doesn't share the same beliefs.
Most often, we lock them because they turn from healthy debate into an all-out war....we lock them to keep some control on the boards.
theres thousands of questions just like it