Every work that effects our union with God in a holy fellowship is a true sacrifice; every work, that is, which is referred to that final end, that ultimate good, by which we are able to be in the true sense happy. As a consequence even that mercy by which aid is given to man is not a sacrifice unless it is done for the sake of God. Sacrifice, though performed or offered by man, is something divine; that is why the ancient Latins gave it this name of “sacrifice,” of something sacred. Man himself, consecrated in the name of God and vowed to God, is therefore a sacrifice insofar as he dies to the world in order to live for God. This too is part of mercy, the mercy that each one has for himself. Scripture tells us: "Have mercy on your soul by pleasing God."
Works of mercy, then, done either to ourselves or to our neighbor and referred to God are true sacrifices. Works of mercy, however, are performed for no other reason than to free us from wretchedness and by this means to make us happy (and we cannot be happy except through the good of which Scripture speaks: "It is good for me to cling to God.") It clearly follows that the whole redeemed city, that is, the assembly and fellowship of the saints, is offered to God as a universal sacrifice through the great high priest, who in the nature of a slave offered even himself for us in his passion, in order that we might be the body of so great a head. He offered this nature of a slave; he was offered in that nature, because in that nature he is the mediator, in that nature he is the high priest, in that nature he is the sacrifice.
The Apostle urges us to present our bodies as "a living sacrifice, holy and pleasing to God, and as our spiritual worship," and not to follow the pattern of this world but to be transformed by the renewal of our minds and hearts, so that we may discern what is the will of God, what is good and pleasing and perfect, the total sacrifice that is ourselves. "By the grace of God that has been given me, he says, I say to all who are among you: Do not think more highly of yourselves than you should, but judge yourselves with moderation according to the measure of faith God has given to each of you. As we have in the same body many members, yet all the members do not have the same functions, we are many, but are one body in Christ; we are each of us members of one another, having different gifts according to the grace that has been given us."
This is the sacrifice of Christians, "the many who are one body in Christ." This is the sacrifice which the Church celebrates in the sacrament of the altar, that sacrament known to the faithful; in that sacrament it is made clear to the Church that in the sacrifice she offers, she herself is offered.
In every place there is sacrifice and a clean oblation offered to my name
From The City of God by Saint Augustine, bishop
The Liturgy of the Hours, Vol. IV, Office of Readings
Friday of the 28th Week of Ordinary Time