Do you have the right to be an atheist?

[Esquilax]

Ah, the old stalwart argument from ignorance: "You don't know everything, therefore god."

It's a non-starter for one simple reason: it assumes possibility.

The OP talks about grains of sand, about computers, and communities and so on, but those are all things that we know are possible, because they are demonstrable. Nobody has been able to ascertain the same about god, and this is a problem because even the set of everything unknown to us only contains things that are possible. Impossible things don't exist, that's just a tautology.

Why does the OP believe in god? "Probability!" he says, and he expects that we'll find this sufficient, but no: for something to be probable, it first has to be possible, and appealing to our lack of knowledge to establish probability without first having a firmly justified notion that the idea if even possible in the first place is skipping the first step. It's a non-starter until possibility can be established.