13 Or if you understand actual men to be meant by children, the words, “If your children will keep My covenant and My testimonies that I shall teach them,” may mean, “If your children will keep My covenant and testimonies that I shall teach them, and their children also;” that is, if they too keep My covenant; so that here you must make a slight pause, and then infer that “they shall sit upon your seat for evermore;” that is, both your children and their children, but all if they keep My covenant.
What then, if they keep it not? Hath the promise of God failed? No: but it is said and promised for this reason, that God foresaw: what, save that they would believe? But that no man should as it were threaten God's promises, and prefer to place in his own power the fulfilment of what God promised: for this reason he says, “He made an oath:” whereby he shows that it will without doubt take place. How then has He said here, “If they will keep My covenant”? Glory not in the promises, and leave out your failing to keep the covenant.
Then will you be the son of David, if you shall keep the covenant; but if you dost not keep it, you will not be David's son. God promised to the sons of David. Say not, I am David's son if you degenerate. If the Jews, who were born of this very stock, say not this (nay, they say it, but they are under a delusion. For the Lord says openly, “If you were Abraham's children, you would do the works of Abraham.” He thereby denied them to be children, because they did not the works), how do we call ourselves David's children, who are not of his race according to the flesh?
It follows then that we are not children, save by imitating his faith, save by worshipping God, as he worshipped. If therefore what you hope not through descent, you will not endeavour to obtain by works; how shall the sitting upon David's seat be fulfilled in you? And if it shall not be fulfilled in you, do you think that it shall not be fulfilled at all? And how has He found it in the woodland tracts? And how did His feet stand? Whatsoever then you may be, that house will stand.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)