GROVE CITY, Ohio — On a recent Saturday in a church fellowship hall here, evangelical Christians from several states gathered for an all-day seminar on how to woo Muslims away from Islam. The teacher urged a kindly approach: always show Muslims love, charity and hospitality, he said, and carry copies of the New Testament to give as gifts. The students, scribbling notes, included two pastors, a school secretary and college students who said they hoped to convert Muslims in the United States, or on mission trips abroad. But although the teacher, an evangelical preacher from Beirut, stressed the need to...