Justin Lee believes that the Virgin birth was real, that there is a heaven and a hell, that salvation comes through Christ alone and that he, the 29-year-old son of Southern Baptists, is an evangelical Christian. Just as he is certain about the tenets of his faith, Mr. Lee also knows he is gay, that he did not choose it and cannot change it. To many people, Mr. Lee is a walking contradiction, and most evangelicals and gay people alike consider Christians like him horribly deluded about their faith. “I’ve gotten hate mail from both sides,” said Mr. Lee, who...