9 But what says he of them? The next words are, “Let them be even as the grass of the house tops: that withers before it be plucked up”. The grass of the house tops is that which grows on house tops, on a tiled roof: it is seen on high, and has not a root. How much better would it be if it grew lower, and how much more joyfully would it bloom? As it is, it rises higher to a quicker withering. It has not yet been plucked up, yet has it withered: not yet have they received sentence from the judgment of God, and already they have not the sap of bloom. Observe their works, and see that they have withered....The reapers will come, but they fill not their sheaves from these. For the reapers will come, and will gather the wheat into the barn, and will bind the tares together, and cast them into the fire. Thus also is the grass of the house tops cleared off, and whatever is plucked from it, is thrown into the fire; because it had withered even before it was plucked up. The reaper fills not his hands thence. His next words are, “Whereof the reaper fills not his hand; neither he that binds up the sheaves his bosom”. And, “the reapers are the angels,” the Lord says.
10. “So that they who go by say not so much as, The blessing of the Lord be upon you: we have blessed you in the name of the Lord”. For you know, brethren, when men pass by others at work, it is customary to address them, “The blessing of the Lord be upon you.” And this was especially the custom in the Jewish nation. No one passed by and saw any one doing any work in the field, or in the vineyard, or in harvest, or anything of the sort; it was not lawful to pass by without a blessing....Who are the passers by? They who have already passed hence to their country through this road, that is, through this life: the Apostles were passers by in this life, the Prophets were passers by. Whom did the Prophets and Apostles bless? Those in whom they saw the root of charity? But those whom they found lifted on high on their house tops, and proud in the bosses of their bucklers, they declared against these what they were doomed to become, but they gave them no blessing. You therefore who read in the Scriptures, find all those wicked men whom the Church bears, who are declared cursed, pertain unto Antichrist, pertain unto the devil, pertain to the chaff, pertain to the tares....But they who say, None save God sanctifies, nor is any man good save by the gift of God; they bless in the name of the Lord, not in their own name: because they are the friends of the bridegroom, they refuse to be adulterers of the bride.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)