legitimate: If you say that something such as a feeling or claim is legitimate, you think that it is reasonable and justified.
Legitimate definition and meaning | Collins English Dictionary
In my opinion, two legitimate reasons not to believe in God are as follows:
1. There is no proof that God exists
2. There is too much suffering in the world for God to exist
I believe there are also legitimate reasons to believe in God as either position can be argued and justified with reason.
As
@Heyo said, no one seems to know what a God actually is.
And that's not simply a problem for the Abrahamic God because there isn't one god, there are thousands, and through history countless thousands of gods.
So I think a very strong argument can be made that gods are something humans devise ─ to explain natural phenomena, like thunder and lightning, meteors, eclipses; and plagues and droughts and floods; and good and bad luck whether in love or war or hunting or fortune; and things that go bump in the night &c. As well, they usually form part of the stories of tribal identity, along with language, customs, heroes and folk histories, all of which promote tribal solidarity hence survival. And you can pray to them, which perhaps gives a sense of control over things that are not otherwise controllable.
But whether that explanation is right, or half-right, or wrong, it remains the case that God never says and never does, and has no description appropriate to a real entity, and that the world behaves just as if God / gods were simply an idea.