Don’t worry about being the best in the world. You’ll probably never get there, and even if you do, the whole process probably won’t be that enjoyable.
Still, do strive to be constantly improving, in at least a couple different areas of life.
By having more than one focus, more than one realm of mastery that is being developed, we have a chance of being “the best” in a different way, in a healthier and more enjoy able way.
Try to be the best at the intersection of two (or more) skills. If your two passions are marathons and chess, then be a better chess player than any other marathoner. There will be marathoners who can run faster, further, stronger than you, but they’ll all know you and respect you as a remarkably skilled chess player. And be the best marathoner of all the chess players. Sometimes you’ll lose at chess, but all of those chess players will know they couldn’t outrun you in a million years.
It is not hard, it seems to me, to be exceptional in one field by mastering another relevant field. If you’re a theologian, study theology fiercely, yes, but study another field as well, economics, let’s say. Get really good at economics on your own time, and be able to speak theologically about economics as no other theologians will be able to do.
Set these sorts of lesser goals. See how much more attainable they are and how much more they will benefit you for the same expenditure of effort.
And as the garden of skills grows, we might become with time well-rounded, knowledgeable generalists. The world could benefit from having a few more folks of that sort, I think.