12 What then? “The Lord has made a faithful oath unto David, and He shall not shrink from it; Of the fruit of your body shall I set upon your seat. If your children will keep My covenant and My testimonies that I shall learn them, their children also shall sit upon your seat for evermore”. If your children keep My covenant, their children also shall sit for evermore. The parents establish a desert on behalf of their children. What if his children should keep the covenant, and their children should not keep it?
Why is the happiness of the children promised in relation to their parents' deservings? For what says He, “If your children will keep My covenant, their children also shall sit for evermore”— He says not, if your children keep My covenant, they shall sit upon your seat; and if their children keep My covenant, they also shall sit upon your seat: but he says, “If your children keep My covenant, their children also shall sit upon your seat for evermore”— except because He here wished their fruit to be understood by their children?
“If your children,” He says, “will keep My covenant, and if your children shall keep My testimonies that I shall learn them; their children also shall sit upon your seat:” that is, this will be their fruit, that they sit upon your seat. For in this life, brethren, do all of us who labour in Christ, all of us who tremble at His words, who in any way endeavour to execute His will, and groan while we pray His help that we may fulfil what He commands; do we already sit in those seats of bliss which are promised us?
No: but holding His commandments, we hope this will come to pass. This hope is spoken of under the figure of sons; because sons are the hope of man living in this life, sons are his fruit. For this reason also men, when excusing their avarice, allege that they are reserving for their children what they hoard up; and, unwilling to give to the destitute, excuse themselves under the name of piety, because their children are their hope. For all men who live according to this world, declare it to be their hope, to be fathers of children they may leave behind them.
Thus then He describes hope generally under the name of children, and says, “If your children will keep My covenant and My testimonies that I shall learn them, their children also shall sit upon your seat for evermore:” that is, they shall have such fruits, that their hope shall not deceive them, that they may come there where they hope to come. At present therefore they are as fathers, men of hope for the future; but when they have attained what they hope, they are children; because they have brought forth and produced in their works that which they gain. And this is preserved unto them for the future, because futurity itself commonly signifies children.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)