Love Thy Stranger: How the Teachings of Jesus Transformed the Moral Conscience of the West

As Bart Erhrman's (relatively) new book, Love Thy Stranger: How the Teachings of Jesus Transformed the Moral Conscience of the West chronicles, loving others without hope of return wasn't really something humanity did before Jesus came along, so modern atheists are conforming to a standard conveyed to them by Christianity when they act thusly.