HONOLULU — President Obama celebrated a low-key Christmas in Hawaii this year. He sang carols, opened presents with his family, and visited a nearby military base to wish the troops “Mele Kalikimaka” — the Hawaiian phrase meaning “Merry Christmas.” But the one thing the president and his family did not do — something they have rarely done since he entered the White House — was attend Christmas church services. “He has not gone to church hardly at all, as president,” said Gary Scott Smith, the author of “Faith and the Presidency: From George Washington to George W. Bush,” adding that...