Humility

The life God bestows is imparted not once for all, but each moment continuously, by the unceasing operation of His mighty power. Humility, the place of entire dependence on God, is, from the very nature of things, the first duty and the highest virtue of the creature, and the root of every virtue.
And so pride, or the loss of this humility, is the root of every sin and evil. It was when the now fallen angels began to look upon themselves with self-complacency that they were led to disobedience, and were cast down from the light of heaven into outer darkness. Even so it was, when the serpent breathed the poison of his pride, the desire to be as God, into the hearts of our first parents, that they too fell from their high estate into all the wretchedness in which man is now sunk