I see what you're saying, but for religious people, their belief are very much a part of what makes them who they are. When you say, "[Fill in the blank with any religious group] are so gullible" or "so self-righteous" or "so controlling" or whatever, everyone in that group feels insulted, even though they may not have the trait that is being criticized at all.
There is definitely a way to express disapproval of certain beliefs or practices without being offensive, but most people don't want to bother to make the effort.