With that being said, being an Atheist doesn't automatically make a person rational.
Being rational opens a person's eyes to the beauty of the universe.
I don't think true rationality is something that happens naturally. Brains are easily deceived. Critical thinking is needed to be rational.
A person has to dedicate a lot of time & energy to find out what is real and how things actually work in real-life.
A person doesn't need to know everything to be an Atheist. Meaning a person doesn't have to know everything to disbelieve in a god or gods. Atheists just go one god further.
It's hard for a lot of people to take that one step further, so they stay an agnostic theist instead of atheist.
For me, Theism is only rational for the people that have mental problems, like they think they would want to do bad things if they don't have a god (in their mind) to punish them.
It is hard to be an honest rational human being for most people. It takes a lot of brain power.