11 Let therefore the slave purchased at so great a price confess his condition, and say, Behold, O Lord, how that I am Your servant: “I am Your servant, and the son of Your handmaid”....This, therefore, is the son of the heavenly Jerusalem, which is above, the free mother of us all. And free indeed from sin she is, but the handmaid of righteousness; to whose sons still pilgrims it is said, “You have been called unto liberty;” and again he makes them servants, when he says, “but by love serve one another.”...Let therefore that servant say unto God, Many call themselves martyrs, many Your servants, because they hold Your Name in various heresies and errors; but since they are beside Your Church, they are not the children of Your handmaid. But “I am Your servant, and the son of Your handmaid.” “You have broken my bonds asunder.”
12. “I will offer to You the sacrifice of praise”. For I have not found any deserts of mine, since You have broken my bonds asunder; I therefore owe You the sacrifice of praise; because, although I will boast that I am Your servant, and the son of Your handmaid, I will glory not in myself, but in You, my Lord, who hast broken asunder my bonds, that when I return from my desertion, I may again be bound unto You.
13. “I will pay my vows unto the Lord”. What vows will you pay? What victims have you vowed? What burnt-offerings, what holocausts? Do you refer to what you have said a little before, “I will receive the cup of salvation, and will call upon the Name of the Lord;” and, “I will offer to You the sacrifice of thanksgiving”? And indeed whosoever well considers what he is vowing to the Lord, and what vows he is paying, let him vow himself, let him pay himself as a vow: this is exacted, this is due. On looking at the coin, the Lord says, “Render unto Cæsar the things which are Cæsar's, and unto God the things which are God's:” his own image is rendered unto Cæsar: let His image be rendered unto God.
14. “In the courts,” he says, “of the Lord's house”. What is the Lord's house, the same is the Lord's handmaid: and what is God's house, save all His people? It therefore follows, “In the sight of all His people.” And now he more openly names his mother herself. For what else is His people, but what follows, “In the midst of you, O Jerusalem”? For than that which is returned grateful, if it be returned from peace, and in peace. But they who are not sons of this handmaid, have loved war rather than peace....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)