In "The Chief Mourner of Marne" by G. K. Chesterton, Mr. Mallow went to Father Brown to consult him about very serious problem, but he found him sitting on the floor with a serious expression, and attempting to pin the somewhat florid hat belonging to a wax doll on to the head of a teddy bear:
A new atmosphere of attention seemed to be created with the mention of the newspaper proprietor. Father Brown neither knew nor cared that his attitudes were comic or commonplace. He continued to sit on the floor, where his large head and short legs made him look very like a baby playing with toys. But there came into his great grey eyes a certain expression that has been seen in the eyes of many men in many centuries through the story of nineteen hundred years; only the men were not generally sitting on floors, but at council tables, or on the seats of chapters, or the thrones of bishops and cardinals; a far-off, watchful look, heavy with the humility of a charge too great for men. Something of that anxious and far-reaching look is found in the eyes of sailors and of those who have steered through so many storms the ship of St. Peter.
What's meant by this, especially "humility" mean "low" or "modest", so how it could be "very great"?