Is this comparison fair?

[Nonentity]

When I say "some 90% of people in the world believe in God or something like God" my friend says "Everyone believed the world was flat. That doesn't make it true." What do I say to that?

[Bryan]

He's right. To suggest that God exists simply because 90% of people believe that he does is to commit a logical fallacy (argumentum ad populum). If you're debating the existence of God with him, I would simply refer him to several of the traditional arguments for his existence (cosmological, teleological, moral, etc.) that are defended on this site.

[FNB - Former non-believer]

Though I would consider that if someone wants to refute what 90% of people believe, you should suspect convincing argument rather than evasions of any burden of proof as if the atheist side was just obvious and needed no argument for its support.

[Nightvid Cole]

Though I would consider that if someone wants to refute what 90% of people believe, you should suspect convincing argument rather than evasions of any burden of proof as if the atheist side was just obvious and needed no argument for its support.

What if the god-beliefs of the believers around the world differ and often contradict? You at least have to admit that the manner in which those individuals came to hold those beliefs are epistemologically unreliable, because by the same mechanisms they draw contradictory conclusions. Even if one particular religion is true, the contradiction shows that the epistemology of the believers in the other religions has to be unreliable, which means that the majority of the believers possess an unreliable epistemology by which they arrived at their conclusions, thus undermining any inference from majority belief to truth.

[Soyeong]

When I say "some 90% of people in the world believe in God or something like God" my friend says "Everyone believed the world was flat. That doesn't make it true." What do I say to that?

Contrary to popular belief, I think there wasn't a time when everyone thought the world was flat, especially by the time of the Middle Ages.  However, your friend's point still stands that a belief being popular doesn't make it true, though I would say that a belief being popular does mean it is more likely to be true.  If you knew nothing about a particular claim except that everyone else on the planet believed it was true, then that should indicate to you that good reasons exist to believe that it is true that are much stronger than the reasons to think it is false.  So even if you don't know what those particular reasons are, you still have reason to think that it is probably true. 

[Nightvid Cole]

When I say "some 90% of people in the world believe in God or something like God" my friend says "Everyone believed the world was flat. That doesn't make it true." What do I say to that?

Contrary to popular belief, I think there wasn't a time when everyone thought the world was flat, especially by the time of the Middle Ages.  However, your friend's point still stands that a belief being popular doesn't make it true, though I would say that a belief being popular does mean it is more likely to be true.  If you knew nothing about a particular claim except that everyone else on the planet believed it was true, then that should indicate to you that good reasons exist to believe that it is true that are much stronger than the reasons to think it is false.  So even if you don't know what those particular reasons are, you still have reason to think that it is probably true.

Yes, but we know more than that - see my above comment. (Not to mention that not everyone else believes such a thing)

[jayceeii]

When I say "some 90% of people in the world believe in God or something like God" my friend says "Everyone believed the world was flat. That doesn't make it true." What do I say to that?

I’d ask the question of why people hearing the idea of God, think they know what a God would be. The presence of a living God may be inaccessible to minds of a certain rank, for instance the humans below the angels. The human tendency is to conclude God will reflect their desires. Instead it can be argued God, and the angels, are desireless entities.

Quite terribly, it may not be possible to communicate that there is a God, to people who are incapable of thinking about God. No matter what is said or what examples are shown, they always turn away sure it must be like a powerful man, a father-figure with standards that are comprehensible to them. The Bible warns otherwise, God’s ways are not man’s.