10 “For the cup in the hand of the Lord of pure wine is full of mixed”. Justly so. “And He has poured out of this upon this man; nevertheless, the dreg thereof has not been emptied; there shall drink all the sinners of earth.” Let us be somewhat recruited; there is here some obscurity....The first question that meets us is this, “of pure wine it is full of mixed.” How “of pure,” if “of mixed”? But when he says, “the cup in the hand of the Lord” (to men instructed in the Church of Christ I am speaking), you ought not indeed to paint in your heart God as it were circumscribed with a human form, lest, though the temples are shut up, you forge images in your hearts.
This cup therefore does signify something. We will find out this. But “in the hand of the Lord,” is, in the power of the Lord. For the hand of God is spoken of for the power of God. For even in reference to men ofttimes is said, in hand he has it: that is, in his power he has it, when he chooses he does it. “Of pure wine it is full of mixed.” In continuation he has himself explained: “He has inclined,” he says, “from this unto this man; nevertheless the dreg thereof has not been emptied.”
Behold how it was full of mixed wine. Let it not therefore terrify you that it is both pure and mixed: pure because of the genuineness thereof, mixed because of the dreg. What then in that place is the wine, and what the dreg? And what is, “He has inclined from this unto this man,” in such sort that the dreg thereof was not emptied?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)