Atheism assumes that God is not real, and claims to be entirely rational. But that second part about, being entirely rational, can be proven to be wrong, simply by thinking it through just once:
So lets assume there is no God. Then the universe at some point just popped into existence right? Of course not. That wouldn't make any sense now would it?
Some scientest might throw in, that the universe itself might be a fluctuation of a quantum field, or simply one of many universes popping out of whatever they pop out.
And of course they could be right, and of course my question to them remains the same: Then did this something that the universe popped out of just suddenly pop into existence? Of course not, that wouldn't make any sense either.
Then this must mean, that this nature (or space or quantum field or whatever unfathomable thing the universe popped out of) has always been there, has always existed, eternally. Existence itself must be eternal then, if it wasn't created by an eternal God. Think it through well. There is no third option, so much is for sure...
Now lets construct an example, that is able to make us see the problem here properly: Imagine you borrowed a bike from your friend. And this friend had borrowed it from another friend. And this guy again borrowed it. And so on and so forth. The bike was given from one person to the next. Reaching back through history, through time.
Let's not complicate things by assuming there was a big bang for the sake of the example though. Let's just assume that this universe itself is eternal and not the quantum field it popped out of.
So since the universe is eternal (and the bike symbolizes existence itself), this chain stretches into the past, without end... It is equally eternal. Now, answer this question: Why is there a bike? How can it always be borrowed, if there is no original owner?
It simply makes no sense to reason, it does not compute in our rational mind. An eternal universe/multiverse is irrational in the very essence of the word.