It all depends how you view god (or your religion), and how one views life.
Plus , there are many forms of mental strength: memory,calculation, imagination, resilience, focus, recognition, analytical, etc.
Just because someone does or does not believe in God, doesn't gauge their mental strength.
Some religions teach curiosity and depth of thought, and some don't.
Some atheists never think, while others continually push themselves to think crucially.
Im not even sure how you came to this question.
Supoosedly God is about belief. Does belief require mental strength?
And if so ....what is the difference between someone who struggles to be good because they fear eternal punishment, and some one who struggles to be good because they believe its the right thing to do ?
Both are mental struggles to achieve the same morals.
