12 “Your kingdom.” What kingdom mean I? “a kingdom of all ages.” For the kingdom of this age too has its own beauty, but there is not in it that greatness of beauty, such as in the “kingdom of all ages.” “And Your dominion is in every generation and generation”. This is the repetition we noticed, signifying either every generation, or the generation which will be after this generation. “Faithful is the Lord in His words, and holy in all His works.” “Faithful is the Lord in His words:” for what has He promised that He has not given?
“Faithful is the Lord in His words.” Hereto there are certain things which He has promised, and has not given; but let Him be believed from the things which He has given. We might well believe Him, if He only spoke: He willed not that we should believe Him speaking, but that we should have His Scriptures in our hands:...as though a kind of bond of God's, which all who pass by might read, and might keep to the path of its promise. And how great things has He already paid in accordance with that bond!
Do men hesitate to believe Him concerning the Resurrection of the dead and the Life to come, which alone now remains to be paid, when, if He come to reckon with the unbelievers, the unbelievers must blush? If God say to you, You have My bond: I have promised judgment, the separation of good and bad, everlasting life for the faithful, and will you not believe? There in My bond read all that I have promised, reckon with me: verily even by counting up what I have paid, you can believe that I shall pay what still I owe.
In that bond you have My only-begotten Son promised, “Whom I spared not, but gave Him up for you all:” reckon this then among what is paid. Read the bond: I promised therein that I would give by My Son the earnest of the Holy Spirit: reckon that as paid. I promised therein the blood and the crowns of the glorious Martyrs; let the White Mass remind you that My debt has been paid....He sets before the eyes of all His payment of His debts: some He has paid in the time of our ancestors, which we saw not: some He has paid in our times, which they saw not; throughout all generations He has paid what was written.
And what remains? Do men not believe Him, when He has paid all this? What remains? Behold you have reckoned: all this He has paid: is He become unfaithful for the few things which remain? God forbid! Wherefore? Because “the Lord is faithful in His words, and holy in all His works.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)