Having fallen from his heavenly rank through pride, the devil constantly strives to bring down also all those who wholeheartedly wish to approach the Lord; and he uses the same means which caused his own downfall, that is pride and love of vainglory. These and similar things are the means by which the demons fight us and hope to separate us from God.
Moreover, knowing that he who loves his brother loves also God, they put into our hearts hatred of one another – and this to such degree that at times a man cannot bear to see his brother or say a word to him. Many have performed truly great labors of virtue, but have ruined themselves through folly. It would not be surprising if the same thing were to happen to you too; if, for example, having cooled towards active work, you begin to imagine that you already possess virtues. For there you have already fallen into that devilish disease (high opinion of yourself), thinking that you are close to God and are in the light, whereas in actual fact you are in darkness.
What made our Lord Jesus Christ lay aside his garments, gird himself with a towel, and, pouring water into a basin, begin to wash the feet of those who were below Him, if not to teach us humility? For it was humility He showed us by example of what He then did. And indeed those who want to be accepted into the foremost rank cannot achieve this otherwise than through humility; for in the beginning the thing that caused downfall from heaven was a movement of pride. So, if a man lacks extreme humility, if he is not humble with all his heart, all his mind, all his spirit, all his soul and body – he will not inherit the kingdom of God.
~ St Anthony the Great, “Early Fathers From the Philokalia,” by E. Kadloubovsky and G.E.H. Palmer, (London: Faber and Faber, 1954), pp. 45-46