A true sacrifice is anything that we do with the aim of being united to God in holy fellowship – anything that is that is directed towards that supreme good and end in which alone we can be truly blessed. It follows that even an act of compassion towards men is not a sacrifice, if it is not done for the sake of God. Although it is performed by man, sacrifice is still a divine thing, as the Latin word indicates: “sacri-ficium”, “holy-doing” or “holy-making”. Man himself can be a sacrifice, if he is consecrated in the name of God, and vowed to God – a sacrifice in so far as he dies to the world in order to live to God....

[Jesus Christ] took on the form of a servant and suffered for us. It was under this form that he both offered and was offered: at the same time mediator, and priest, and sacrifice.

St Paul starts by exhorting us to present our bodies as a living sacrifice, holy and acceptable to God, as an act of homage justly owed to him. He tells us not to con-form ourselves to the world but to be trans-formed by renewing our will and our thinking: seeking to find out the will of God, to discover what is good, what is acceptable, what is perfect; for we ourselves are the whole of that sacrifice....

This is the sacrifice of Christians: we, being many, are one body in Christ. And, as the faithful know, this also is the sacrifice which the Church continually celebrates in the sacrament of the altar, in which she teaches that she herself is offered in the offering she makes to God.

(St Augustine, City of God)