Most of the spate of Christian-themed films that have appeared in the past decade have been serious in nature, so it is refreshing to see The Resurrection of Gavin Stone (Universal, “PG,” 184 min., $26.98, BD $29.98), a sweet little comedy about a washed-up, partied-out Hollywood child star who is reduced to pretending to be “born again” in order to snag the role of Jesus in a mega-church’s elaborate portrayal of the nativity.