2 Whence then does He begin? And what kind of foundations of His new polity does 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. Thus He said not, “Blessed are you, if you become poor,” but “Blessed are the poor.” And I may add that even if He had spoken of them, the advice would still be common to all. For so, when He says, “Lo! I am with you always, even unto the end of the world,” He is discoursing not with them only, but also, through them, with all the world. And in pronouncing them blessed, who are persecuted, and chased, and suffer all intolerable things; not for them only, but also for all who arrive at the same excellency, He weaves His crown.
However, that this may be yet plainer, and to inform you that you have great interest in His sayings, and so indeed has all mankind, if any choose to give heed; hear how He begins these wondrous words.
“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 has 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 trembles 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 says, “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.
Source: Homilies on the Gospel of St. Matthew (New Advent)