15 That happy man, then, will become like this tree when he shall be transplanted, as the thief was, into the garden and set to grow beside the rills of water: and his planting will be that happy new planting which cannot be uprooted, to which the Lord refers in the Gospels when He curses the other kind of planting and says: Every planting that My Father has not planted shall be rooted up. This tree, therefore, will yield its fruits. Now in all other passages where God's Word teaches some lesson from the fruits of trees, it mentions them as making fruit rather than as yielding fruit, as when it says: A good tree cannot make evil fruits, and when in Isaiah the complaint about the vine is: I looked that it should make grapes, and it made thorns. But this tree will yield its fruits, being supplied with free-will and understanding for the purpose. For it will yield its fruits in its own season. And, pray, in what season? In the season, of course, of which the Apostle speaks: That He might make known unto you also the mystery of His Will, according to His good pleasure which He has purposed in Himself, in the dispensation of the fullness of time. This, then, is the dispensation of time, by which is regulated the right moment of receiving, in the case of the recipients, and of giving, in that of the giver; for the giver has choice of the season. But delay in point of time depends upon the fullness of times. For the dispensation of yielding fruit waits upon the fullness of time. Now what, you ask, is this fruit that is to be dispensed? That assuredly of which this same Apostle is speaking when he says: And He will change our vile body, that it may be fashioned like His glorious body. Thus He will give us those fruits of His which He has already brought to perfection in that man whom He has chosen to Himself who is portrayed under the image of a tree, whose mortality He has utterly done away and has raised him to share in His own immortality.
This man then will be happy like that tree, when at length he stands surrounded by the glory of God, being made like the Lord.
Source: Homilies on the Psalms (New Advent)