Matthew 5:3

(De Officiis i. 16.) In the eye of Heaven blessedness begins there where misery begins in human estimation.

Whence then doth He begin? and what kind of foundations of His new polity doth He lay for us? Let us hearken with strict attention unto what is said. For though it was spoken unto them, it was written for the sake also of all men afterwards. And accordingly on this account, though He had His disciples in His mind in His public preaching, yet unto them He limits not His sayings, but applies all His words of blessing without restriction. For He said not, This or that person, but they who do so, are all of them blessed. So that though thou be a slave, a beggar, in poverty, a stranger, unlearned, there is nothing to hinder thee from being blessed, if thou emulate this virtue. Blessed are the poor in spirit; for theirs is the kingdom of Heaven. What is meant by the poor in spirit? The humble and contrite in mind. For by spirit He hath here designated the soul, and the faculty of choice. That is, since many are humble not willingly, but compelled by stress of circumstances; letting these pass (for this were no matter of praise), He blesses them first, who by choice humble and contract themselves. But why said he not, the humble, but rather the poor? Because this is more than that. For He means here them who are awestruck, and tremble at the commandments of God. Whom also by His prophet Isaiah God earnestly accepting said, To whom will I look, but to him who is meek and quiet, and trembleth at My words? For indeed there are many kinds of humility: one is humble in his own measure, another with all excess of lowliness. It is this last lowliness of mind which that blessed prophet commends, picturing to us the temper that is not merely subdued, but utterly broken, when he saith, The sacrifice for God is a contrite spirit, a contrite and an humble heart God will not despise. And the Three Children also offer this unto God as a great sacrifice, saying, Nevertheless, in a contrite soul, and in a spirit of lowliness, may we be accepted. This Christ also now blesses. For whereas the greatest of evils, and those which make havoc of the whole world, had their entering in from pride - for both the devil, not being such before, did thus become a devil; as indeed Paul plainly declared, saying, Lest being lifted up with pride, he fall into the condemnation of the devil - and the first man, too, puffed up by the devil with these hopes, was made an example of, and became mortal (for expecting to become a god, he lost even what he had; and God also upbraiding him with this, and mocking his folly, said, Behold, Adam is become as one of us); and each one of those that came after did hereby wreck himself in impiety, fancying some equality with God - since, I say, this was the stronghold of our evils, and the root and fountain of all wickedness, He, preparing a remedy suitable to the disease, laid this law first as a strong and safe foundation. For this being fixed as a base, the builder in security lays on it all the rest. But if this be taken away, though a man reach to the Heavens in his course of life, it is all easily undermined, and issues in a grievous end. Though fasting, prayer, almsgiving, temperance, any other good thing whatever, be gathered together in thee; without humility all fall away and perish. It was this very thing that took place in the instance of the Pharisee. For even after he had arrived at the very summit, he went down with the loss of all, because he had not the mother of virtues: for as pride is the fountain of all wickedness, so is humility the principle of all self-command. Wherefore also He begins with this, pulling up boasting by the very root out of the soul of His hearers.

What then, saith one, is the head of virtue? Humility. Wherefore Christ also begins with it, saying, "Blessed are the poor." This head hath not locks and ringlets, but beauty, such as to gain God's favor. For, "Unto whom shall I look," saith He, "but unto him that is meek and humble, and trembleth at my words?" And, "Mine eyes are upon the meek of the earth." And, "The Lord is nigh unto them that are of a contrite heart." This head, instead of locks, and flowing hair, bears sacrifices acceptable to God. It is a golden altar, and a spiritual place of sacrifice; "For a contrite spirit is a sacrifice to God." This is the mother of wisdom. If a man have this, he will have the rest also.

The poor in spirit are those who embrace a voluntary poverty for the sake of the Holy Spirit.

(De Civ. Dei, xix. 1.) The chief good is the only motive of philosophical enquiry; but whatever confers blessedness, that is the chief good; therefore He begins, Blessed are the poor in spirit. (Id. de Serm. in Mont. i. 1.) Augmentation of 'spirit' generally implies insolence and pride. For in common speech the proud are said to have a great spirit, and rightly—for wind is a spirit, and who does not know that we say of proud men that they are 'swollen,' 'puffed up.' Here therefore by poor in spirit are rightly understood 'lowly,' 'fearing God,' not having a puffed up spirit. (ubi sup.) The proud seek an earthly kingdom, of the humble only is the kingdom of Heaven.