"The first food, then, is humility: bitter but medicinal; the second is charity: sweet and soothing; the third is contemplation: solid and strength giving." Source: St. Bernard of Clairvaux, The Steps of Humility and Pride (Note: This quote was incorrectly attributed to St. Augustine when it was initially posted here.)
"Many say: 'I have labored for years for others and even for God, and what did I get out of it? I am still nothing.' The answer is, they have gained something; they have gained the truth of their own littleness – and of course, great merit in the next life." Source: Ven. Fulton [...]
"The devil makes us believe that we have some virtue – patience, let us say – because we have determination and make continual resolutions to suffer a great deal for God’s sake. We really and truly believe that we would suffer all this, and the devil encourages us in the belief, and so we are [...]
"Humility makes us receptive to giving of others. You could not give unless I took. It is the taker that makes the giver." Source: Ven. Fulton J. Sheen, Way to Happiness, St. Pauls, 1998.
"For the person who is truly humble is always doubtful about his own virtues; very often they seem more genuine and of greater worth when he sees them in his neighbors." Source: Saint Teresa of Avila, The Way of Perfection, Dover Thrift Editions, 2012.
"To define humility: Humility is a virtue by which a man has a low opinion of himself because he knows himself well. This is the virtue that belongs to those who have set their hearts to the climb and have gone from virtue to virtue, from step to step, until they reached the highest peak [...]
"Humility is truth about ourselves." Source: Ven. Fulton J. Sheen, Way to Happiness, St. Pauls, 1998.
"You must not think that you have made any progress until you look upon yourself as inferior to all others." Source: Thomas a’ Kempis, The Imitation of Christ, Dover Thrift Editions, 1940.
"How much happier people would be if instead of exalting their ego to infinity, they reduced it to zero." Source: Ven. Fulton J. Sheen, Way to Happiness, St. Pauls, 1998.
"Without humility, we cannot be pleasing to God. Practice the third degree of humility; that is, not only must one refrain from explaining and defending oneself when reproached with something, but one should rejoice at the humiliation." Source: St. Faustina, Diary: Divine Mercy in My Soul