3 But wonderful it is, that when Christ Crucified is preached, two hear, one despises, the other ascends. Let him that despises, impute it to himself; let not him that ascends, arrogate it to himself. For he has heard from the True Master; “No man comes unto Me, except it were given unto him of My Father.” Let him joy, that it has been given; let him render thanks to Him who gives it, with a humble, not an arrogant heart; lest what he has attained through humility, he lose through pride.
For even they who are already walking in this way of righteousness, if they attribute it to themselves, and to their own strength, perish out of it. And therefore Holy Scripture teaching us humility says by the Apostle, “Work out your own salvation with fear and trembling.” And lest hereupon they should attribute ought to themselves, because he said, “Work,” he subjoined immediately, “For it is God who works in you both to will and to do of His good pleasure.” “It is God who works in you;” therefore “with fear and trembling,” make a valley, receive the rain.
Low grounds are filled, high grounds are dried up. Grace is rain. Why do you marvel then, if “God resist the proud, and gives grace unto the lowly”? Therefore, “with fear and trembling;” that is, with humility. “Be not high-minded, but fear.” Fear that you may be filled; be not high-minded, lest you be dried up.
Source: Sermons on Selected Lessons of the New Testament (New Advent)