Found in a lovely article called Conflict: The Birthing of the New by Caroline Westerhoff:

Aquinas defined humility as the patient pursuit of one's own excellence. We are called by God's grace who we already are, and if we and we alone can stand squarely on our little or large pieces of ground--wherever or whatever they turn out to be--then these spots will be empty if we are not there. They are holy ground, because God made them too, and ordained them so. Whatever is to occur there is important, and we are at least partially responsible for its happening. We are called to answer the divine invitation, to be there, to act, and no one can do it for us.