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.
[…] 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.
God earnestly accepted these by His prophet Isaiah, saying, “To whom will I look, but to him who is meek and quiet, and trembleth at My words?” (Isa. 66:2 [LXX]).
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” (Ps. 50:17).
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” (Dan. 3:39 [LXX]). This Christ also now blesses.
The greatest of evils, and those which make havoc of the whole world, had their entering in from pride.
For 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” (1 Tim. 3:6).
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 upbraided him with this, and mocking his folly, said, “Behold, Adam is become as one of us” (Gen. 3:22).
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.
John Chrysostom (c.347-407): Homilies on the Gospel According to St Matthew, 15, 2-3 (on Matthew 5:3); slightly adapted.