There are plenty of thoughtful atheists and plenty of thoughtful Christians.
Even if there are thoughtful Christians, I think that this is the
main difference.
I don't think that you can pin down the difference to any one, single factor. There are going to be several, and much will depend on just where one's thoughts go.
Thoughtfulness turns theists into atheists. Not reliably. Not all of the time. But that is a major factor.
Not all atheists are produced by thoughtfulness. Some people are simply raised without any encouragement to believe in deities. They could be very unthoughtful, and still be atheists.
And, I will accept the word of Christians here that believing theists can be thoughtful people. These are perhaps atheists waiting to happen, but there is never any guarantee.
So, the idea that there is only
one difference that divides theists and atheists is a fool's quest, IMJ.
My position is that the difference is emotional. Believers want to believe and non-believers do not want to believe.
That is certainly wrong.
I never did become an atheist because I didn't want to believe. It wasn't a matter of desire, but of what was convincing or not. Theism had failed to convince me and retain its foothold in my belief structure. Once it was gone, it was gone. Desire was irrelevant throughout the process.
There was never any choice in the matter except to think about the issues, or not to think about them. I chose to think. Atheism was perhaps not a guaranteed result, but it was the result, and it happened without any specific choice or desire to become an atheist. I was actually quite happy with the Catholicism that I had believed in at the time.
eudaimonia,
Mark