Psalms 54:6

For among them (the Egyptians) these offices, which we are taught to render to the Lord at separate hours and at intervals of time with a reminder from the convener, are celebrated freely and continuously throughout the whole day in conjunction with their work. For manual labor is continuously practiced by them in their cells in such a way that meditation on the Psalms and the rest of the Scriptures is always before them. To this at every moment they mingle short petitions and prayers, spending the whole day in those offices which we celebrate at fixed times. Thus, except for vespers and nocturns, there are no public services among them in the day except on Saturday and Sunday, when they meet together at the third hour for the purpose of Holy Communion. For what is continuously offered is greater than what is rendered at intervals of time. And a free gift is more acceptable than the duties which are performed by the compulsion of a rule. For such as this, David rejoices somewhat exultingly when he says, "Freely will I sacrifice to you;" and, "Let the free will offerings of my mouth be pleasing to you, O Lord."

Listen to the prophet's voice: "I will freely sacrifice to you, O Lord." Learn how different an imposed servitude is from a voluntary one. A person who finds his own negligence accusing himself of suffering self-imposed servitude can never pass a day without regret. A …

"Voluntarily I will sacrifice to You" [Psalm 54:6]. Who can even understand this good thing of the heart, at another's speaking thereof, unless in himself he has tasted it? What is, "Voluntarily I will sacrifice to You"?... For what sacrifice here shall I take, brethren? Or what worthily shall I offer to the Lord for His mercy? Victims shall I seek from flock of sheep, ram shall I select, for any bull in the herds shall I look out, frankincense indeed from the land of the Sabæans shall I bring? What shall I do? What offer; except that whereof He speaks, "Sacrifice of praise shall honour Me"? Wherefore then "voluntarily"? Because truly I love that which I praise. I praise God, and in the self-same praise I rejoice: in the praise of Himself I rejoice, at whom being praised, I blush not. For He is not praised in the same manner as by those who love the theatrical follies is praised either by a charioteer, or a hunter, or actor of any kind, and by their praisers, other praisers are invited, are exhorted, to shout together: and when all have shouted, ofttimes, if their favourite is overcome, they are all put to the blush. Not so is our God: be He praised with the will, loved with charity: let it be gratuitous (or voluntary) that He is loved and that He is praised. What is "gratuitous"? Himself for the sake of Himself, not for the sake of something else. For if you praise God in order that He may give you something else, no longer freely do you love God. You would blush, if your wife for the sake of riches were to love you, and perchance if poverty should befall you, should begin to think of adultery. Seeing that therefore you would be loved by your partner freely, will you for anything else love God? What reward are you to receive of God, O covetous man? Not earth for you, but Himself He keeps, who made heaven and earth. […]

"I will sacrifice willingly to you." Here he sets forth the recompense. And first the recompense is set forth. Second, the reason for the recompense. He intends to offer as recompense two things: sacrifice and confession. With regard to the first he says, "I will …

I will freely sacrifice unto thee,.... Not legal sacrifices; no, nor freewill offerings the law gives directions about, though the allusion is to them; but the freewill offerings of his mouth, Psa 119:108; the sacrifices of praise and thanksgiving, on account of help, salvation, and deliverance, as appears from the following clause; which he determines to offer, not by constraint, but willingly; not by force, but of a ready mind; freely, and with all his heart.