If all the believers went "poof" Peace would reign on earth.
Do you really believe that? Atheists do not "war" or fight? They never argue? All have the same moral values?
What about other "religious" people who do not believe in Jesus Christ as their Lord and Savior? And/or what about all the muslims? They'll still be here; so atheists and muslims would never , ever "war'? Or will the nations be diivided into "what we believe" nations - ones who have Sharia law, and ones with no laws at all for the atheists? Who gets to decide who are the kings of these nations? Who will be kings of these nations?
Who's in authority?
People war because of wanting plain "power" also; not just what they believe in as their faith. We have a sin nature, a fallen nature.
That's the whole reason that Jesus Christ came to save us. From ourselves/sin.
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