Many college students go through a spiritual crisis but rarely does it turn out as it did for Daveed Gartenstein-Ross, a Jew who converted to Islam and went to work in a small town in Oregon for a charity that has since been linked to Al Qaeda. Gartenstein-Ross, 30, has now changed tacks again, converting to Christianity and using his background as a former Islamist insider to help the Federal Bureau of Investigation crack down on Islamist terror networks in the United States. A rising star in the counterterrorism community, he testified before Congress in September about the dangers of...