I think you are approximately right here, but it’s important to think about just how high that upper bound is, and what activities can only be accomplished by people above that bound. It might help to think in more concrete terms about what someone who believes in religion cannot achieve, that a non-believer can.
With sufficient compartmentalization of religious beliefs, I would venture to say the answer is a pretty small subset of activities. They may be important activities on a global scale, but mostly unimportant in peoples’ day to day functioning.
It’s very easy to imagine, or better yet, meet, theists who are far more rational in achieving their goals than even many of the people on this board.
One should also know everything, but clearly that’s impossible.
There are some areas of knowledge that are so unlikely to yield anything useful that it’s not worth spending any time being curious about them. For humanity in general, psi phenomena now fall into this category. There was a time when they didn’t, but it’s safe to say that time is over. For me as an individual, string theory falls into that category. I’m glad there are some people investigating it, but the effort required for me to have anything but a superficial understanding of the topic is extremely unlikely to help me achieve anything.