That’s the question you should always ask believers when they make an unsupported assertions, ranging from “God is loving” to “Our souls live on after death.” The answer will always be one of two things: “The Bible says so,” or “I just know it to be true.” Neither of those are rational answers, but they satisfy the religious.
It is in fact the “how-do-you-know-that” query that really distinguishes New Atheism from Old. While atheists have always decried the lack of evidence for theism, it is the infusion of scientists and science-friendly people into atheism, starting with Carl Sagan and continuing on to Dawkins, Hitchens, Harris, Pinker, and Dennett, that has made us realize that religious dogmas are in fact hypotheses, and you need reasons and evidence for accepting them. If you have none, then you have no reason to believe in God.
Nevertheless, religious dogma does change, but not because theology has found better reasons. It’s because a.) science has shown the dogma to be false (Genesis, Adam and Eve, creation, the Exodus, etc.) or b.) secular morality has shown that the tenets of religious belief are no longer supportable (hell as a place of fire, limbo, discrimination against gays, the Mormons’ refusal to let black be priests, etc.)
But I fulminate. The best “how do you know that” moment of the month is one I learned from a Sky News article: “Angels exist but have no wings, says Church.” In it, we learn that a prescient priest has decided that angels, like worker ants, are wingless:
Angels really do exist but do not have wings and are more like shards of light, according to a church official.
Catholic Church “angelologist” Father Renzo Lavatori [JAC: great name!] says the celestial beings are back in vogue thanks to various New Age religions.
But he insists that the traditional portrayal of angels as hovering, winged cherubs rather misses the mark.
“I think there is a re-discovery of angels in Christianity,” Father Lavatori said at a conference on angels at a lavishly-frescoed Renaissance palace in Rome.