This is the most confusing part of Christianity. God won't answer me unless I accepted His son, which is/was Himself!
I think we can be too dogmatic on this sometimes.
We cannot FORCE God to do anything at all. However, it is not impossible that He can answer, in any way He chooses, any one He chooses, at any time He chooses. Even those who do not believe in Him. (I know some people will disagree with me.)
But I've talked to too many people, seen too much, read biographies of people who had no way of knowing Him otherwise. If we say it's impossible, we limit God.
Truly, the instances of Him speaking in audible words from the heavens are VERY limited, and we should not expect this. But He CAN and has done it. Another reason we cannot limit Him as to what He is capable of doing.
One who wants to taunt and test God, and plays around with calling out and being smug in their own minds when He doesn't answer, is almost certainly never going hear from God in any way. He will not be mocked.
However, one who sincerely, truly, wants to know the truth and seeks Him, who comes to Him with a contrite heart and desires to know Truth - I believe He will make himself known to those, in whatever way is best according to His wisdom. This is the nature of God, and He knows the hearts and thoughts of man.
Oh, and as far as that one or two billion years' prediction squaring with Scripture, Jesus said no one knows the DAY or HOUR. I don't think they've got it pinned down that close.

And I don't think it will ever happen. I believe God will intervene before that time. But that whole point is irrelevant, except for the part about Scripture being contradicted.
