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.
14. “For the Lord has chosen Sion to be an habitation for Himself”. Sion is the Church Herself; She is also that Jerusalem unto whose peace we are running, who is in pilgrimage not in the Angels, but in us, who in her better part waits for the part that will return; whence letters have come unto us, which are every day read. This city is that very Sion, whom the Lord has chosen.
15. “This shall be My rest for ever”. These are the words of God. “My rest:” I rest there. How greatly does God love us, brethren, since, because we rest, He says that He also rests! For He is not sometimes Himself disturbed, nor does He rest as we do; but He says that He rests there, because we shall have rest in Him. “Here will I dwell: for I have a delight therein.”
16. “I will bless her widow with blessings, and will satisfy her poor with bread”. Every soul that is aware that it is bereft of all help, save of God alone, is widowed. For how does the Apostle describe a widow? “She that is a widow indeed and desolate, trusts in God.” He was speaking of those whom we all call Widows in the Church. He says, “She that lives in pleasure, is dead while she lives;” and he numbers her not among the widows. But in describing true widows, what says he? “She that is a widow indeed and desolate, trusts in God, and continues in supplications and prayers night and day.” Here he adds, “but she that lives in pleasure, is dead while she lives.” What then makes a widow? That she has no aid from any other source, save from God alone. They that have husbands, take pride in the protection of their husbands: widows seem desolate, and their aid is a stronger one. The whole Church therefore is one widow, whether in men or in women, in married men or married women, in young men or in old, or in virgins: the whole Church is one widow, desolate in this world, if she feel this, if she is aware of her widowhood: for then is help at hand for her. Do ye not recognise this widow in the Gospel, my brethren, when the Lord declared “that men ought always to pray and not to faint”? “There was in a city a judge,” He said, “which feared not God, neither regarded man. And there was a widow in that city; and she came unto him day by day, saying, Avenge me of mine adversary.” The widow, by daily importunity, prevailed with him: for the judge said within himself, “Though I fear not God; neither regard man, yet because this woman troubles me, I will avenge her.” If the wicked judge heard the widow, that he might not be molested; hears not God His Church, whom He exhorts to pray?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)