Della’s father implies that Jack should be grateful because if he
weren’t a Christian, he’d presumably do Jack harm. Jack was stuck in an awful bind—he loved her, but he also knew Della, and their soon-to-be-born child, really would be better off with her family in many ways. He also knew that if he could genuinely become a Christian, Della’s father would be more open to their relationship. The fact that he refuses to fake a conversion shows that Jack really does have integrity.