I was just in a debate with a christian

[Sword of Christ]

This is as succinct a summation of your arguments so far as I have seen. You are right, you don't know it's true.

That's why it's called a faith but it's based on rational arguments and evidence. Not scientific proof as you can't ever have that but there is scientific evidence, you have the big bang (moment of creation when time itself began to exist as Biblical theism stated) and the fine tuning/complexity of a life sustaining universe. So even science will point toward God or a purposeful intelligent agency as well as something timeless and non-physical. Masses of other arguments and evidences I have only scratched surface on here but if you're interested in finding more about this for yourself it is all there and you can find out about all this.

It follows that you cannot demonstrate it.

You can't demonstrate atheism or even provide decent arguments to support it as being true. You can point out that you can't ever know for certain that God exists or not and you can point out there is no scientific proof but that's not an argument.

You cannot prove the thing you believe in.

You can prove the sheer likelihood of it based on the evidences we have and reasoned arguments, and that's enough to have faith in it than not. You can't prove it with science if God is beyond what science can study and detect so that isn't an issue.

Therefore, I have no reason to believe it, as you cannot demonstrate it.

Atheism is destroyed on the basis of the arguments from morality and freewill alone let alone everything else you can bring to bear, the teleological arguments, arguments from first cause and the universality of belief and so on. All this business involving infinite multiple universes that create each other with blackholes or whatever is a complete crock and there's no evidence for it anyway.

If you have nothing at all to base this upon then it's as good as a blind faith in something you can't know. Particularly if you want to state that's "the truth", the truth because it's your own opinion? That's not how it works.

You're the one who believe in this multiple universe business (something science could demonstrate if it exists) for no other reason than because that's what you want to believe. It's a faith and there is no reason for it hence a blind faith with no justification. It's a man made religion of pure unadulterated madness that's what it is. You must free your mind!

My "basis" for my atheism is that I could not produce the god I believed in.

No you have a God it's just the pure madness of over inflated human egotism thinking it is God is the supreme unintelligence of the universe and knows or will know everything there to know about everything. The arguments and rational arguments for the other kind of God destroy this God you believe in.

So I stopped believing in him because, as you note, my opinion wasn't going to make him exist.

Damn right it doesn't but you can have a right opinion and a wrong opinion. You have an opinion that's really seriously wrong but I have something more than my opinion to counter it, the arguments from reason and various evidences for God do exist. Atheism has nothing but opinion, it's the opinion that there isn't a God or a good reason to believe God exists and that's it.

So it follows that your opinion, backed by the opinion of others, is no more convincing. Especially when you admit that you cannot produce the god that you opine exists.

You just have to refute atheism/materialism and by God that's easy enough.