First you would have to kill God.
I think that is a good thing actually.
And that's just it. How many years have people tried to kill the Jews despite an overwhelming numerical advantage by Jew-haters to the number of Jews? God's not going to let that happen because He still has a plan to save a remnant of them.
Likewise, even if most Christians turn their backs on Christ, God will still have His select for the time of Armageddon and they will rise up when He needs them to win the battle. If Christians wither and fall away, it only means we are drawing closer to that time when Christ returns and God has his final battle with Satan - which He'll win.
Really, the Revelations of John spelled all this out centuries ago and there's no reason to believe it won't happen. What is impossible for God?