Paula, among many others, talks about spirituality and martial arts. This may not be the spiritual aspect she means, but certainly martial arts teach humility when you come up against a compatriot who can do something so much better than you.
For me, in grappling, it's my buddy Larry. Larry by his own admission gets nervous at the black belt test, and so hasn't done well on defense moves that are memorized. But he knows so much about grappling--he knows so many moves I've never seen, and remembers more of the ones we've both been taught--that it's a real exercise in humility grappling with him. I can't blame an age or size difference, he's maybe two or three years younger than me, and the same size basically. He even had ACL reconstruction before I did.
Moreover, he does his excellent work with a modest and unassuming attitude, which itself is a commendable spiritual discipline.