A scientific reason to not believe?

[Angrboda]

Can anyone give me a scientific reason not to be a "believer"?

A Scientific reason: the observable fact that there is more than one religion.
These religions are, for the most part (at least) mutually exclusive. That means that, at best, only a small set of religions can be true.
Which means that all the others are wrong.
How did these come to be? hmmm... human imagination! There's no other way.

How do we figure out which religion, if any, is the correct one?
As far as I'm aware, we can't.
- Number of believers is just a measure of how gullible people are, or how aggressively the belief in that particular deity was enforced.
- Written word... completely falsifiable by humans.

- I'd expect some actual interaction of this deity with our world, but we see no such interaction.

If we can't decide on which religion is true, then.... the most honest option is that all are likely false and man-made!

You neglect the much larger class of religions which nobody has invented, described or endorsed, but which hit on all the same cylinders as the exemplars provided. This class is not necessarily infinite, but it is so uncountably huge that the probability of any one religion - known and/or imagined, or neither - being the correct one, converges on zero pretty dramatically. If you were a betting man, you'd be more likely to collect on a wager that you'd be hit by lightning three times on your way to work than that you've chosen the right one. Yet people still leave their houses, and still find reasons to believe.