He has invoked the Rev. Rick Warren, a popular evangelical author and megachurch pastor. He has quoted Scripture and alluded to the Gideon Bible as favorite late-night reading. And he has cited his belief in Jesus Christ as his personal “savior.” As Mitt Romney has had to grapple with suspicions about his Mormon religion during his presidential run, he has tried in various ways to signal his kinship with evangelical Christians, who represent a crucial constituency of the Republican base but consider his religious beliefs to be heretical. He faces a delicate task in trying to stake out common ground...