That would make 90%+ of the worlds population mentally ill. I think most religious people are very capable of rational thoughts but they manage to suspend what they know to place belief in the unknown.
Obviously, people who use their religion to kill and abuse are obviously mentally ill.
No. That position is espoused only by those who have an anti-religious sentiment.
Humans have natural tendencies to believe in gods and an afterlife that's why there has never been a society that lacks a religion or a category equivalent to the supernatural.