everyone let him decide what to believe him. Why do something to try to prove to atheists? it is their opinion and choice, let them live as they want.
Because we love all people. We love atheists. We don't want to let atheists go to Hell without, at least, giving them a chance to repent. Of course, if they don't repent, Hell will probably be worse for them than if we had never talked to them.
but what about all the other gods? Gods from older religions? Were they wrong and false gods? What makes you think your god is the true one? Don't argue with religious fanatics, they will always use their holy book for "proof"
All gods are false gods in my book, you're talking to an atheist after all, I have no god.
To use an Andrew Dice Clay line, arguing with religious fanatics is like masturbating with a cheese grater, slightly amusing but mostly painful.