Well, we have this thing called "free agency". We can do whatever we want. It is not suggested to read or do anything that can hurt us spiritually. But, I have, yes I admit, and I do have friends who have left the Church. Many members have heard a lot of the ex-mormon propaganda. And a lot of us realized it's just that---propaganda.
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Some Mormons left for theological reasons or disillusionment from authority or history. But I have never found a legitimately reason to leave the Church or live in "unbelief".