Swerving into a Crisis of Faith

Some people walk in the way of truth and then swerve violently for no apparent reason. There is always more going on beneath the surface than any of us can see. I once had a friend who taught others God's word, then over a period of time had a crisis of faith where he basically said he did not believe anymore. What brought it on? Only God knows. I am thankful God is sovereign and has it all under control.