I always ask them.. What about the millions of years before the Jesus story?
Even if they are not creationists why would they care what happened before Jesus?
Secular historians on religion claim that religious belief goes back 40,000 years on this planet. That 40 times a thousand. Detailed history on religion goes back 10,000 years. So what God and his brat were waiting for, I have no earthly idea. Presumably all who lived before Christianity went to hell, no exceptions or exclusions, since all their beliefs were wrong. As for asking Christians they neither know nor care. They live in a time bubble where everything before Jesus popping up is irrelevant (well, okay, maybe Moses et.al) and that's good enough for them.
And what answers have you gotten?
I've heard the bit about the covenant with Abraham or, if you want to skip ahead a bit, the law of Moses. I've also heard the young-Earth story, in which there weren't millions of years - only a few thousand.