I would like to know how Iona can make the claim, "Without Christianity, there can be no evil, no good,"

Because without Christianity you have no foundation to believe anything. You are on the horns of a dilemma. What is evil? I think it was you, Don, who said "Good is what promotes life", or something to that effect. Okay, so how do you know that life is good? It all comes back on what your foundation is. You believe that the world has no author (no divine Author, anyway), and that the world is all a random chance accident. If that is the case then your mind is also merely a random chance accident and you can't be sure that reality is what you see. For all you know, what you call 'evil' is really the quintessential good for another person. And you have no place to judge them, because good and evil must be relative. And since most atheists are materialistic (believing that only what is material exists), you can't even believe in mind. All you have are chemical reactions in your brain.
I say that without Christianity you have no basis to believe anything. I'm not saying "Without religion", I'm saying without Christianity. Other religions are just as impossible to justify as atheism. (though on a different basis.)

Iona