Commenting on Ps 100[101], 1: “I will sing to you of mercy and judgment, O Lord”:

Since mercy and judgment are sung to us, let us do the works of mercy and we can safely await judgment. Let us be among his members and let us sing these things, for Christ is singing them. If only the head is singing, then this song is the Lord’s and it doesn’t concern us. But if the whole Christ, that is, head and members, is singing, then be among his members, cling to him by faith and by hope and by love. Then in him you too are singing; in him you are exulting, because he too toils in you, thirsts in you, hungers in you, is in distress in you; he is still dying in you, and you, already, have risen in him. If he were not dying in you, he would not wish to be spared by the persecutor when he says, “Saul, Saul, why are you persecuting me” (Acts 9:4).

So, brothers and sisters, Christ is singing, and you know how he is doing so because I have often told you, and I know these things are not unfamiliar to you. Christ the Lord is the Word of God through whom all things were made (Jn 1:3, 14). That Word, in order to redeem us, became flesh and dwelt among us (Jn 1:3, 14). God above all things, the Son of God, equal to the Father, became man. He became man so that he might be the God-man mediator between human beings and God and might reconcile those long distant and might re-unite those separated and might call back those alienated and might lead back those living abroad: for all this was he made man. He became the head of the Church and so has a body and members. Look for his members: groaning now throughout the world, they will rejoice in the end, in the crown of righteousness that,” Paul says, “the Lord, just judge, will give to me on that day.”

All of us, then, gathered into one, let us sing in hope. Having put on Christ, we are Christ with our head.... We belong to Christ, and because we are his members and his body, with our head we are one person. Let us sing, then: “I will sing to you of mercy and judgment, O Lord.” (EP 100[101], 3; PL 37, 1282-85)