Because these things happen by the grace of God through Jesus Christ our Lord, in this prophecy the Savior himself adds his voice to that of his Body. I think it’s to its Head himself that what follows belongs: “I am a sharer with all those who fear you and keep your commandments” (Ps 118:3) Just as it’s written in the Epistle to the Hebrews: “The one who sanctifies and those who are sanctified are all of one, and for this reason he is not ashamed to call them his brothers and sisters.” And a little later: “Because the children are sharers of flesh and blood, he also in like manner has shared in them” (Heb 2:11, 14). What does this mean if not that he became a sharer of them. And we would not have been made sharers in his divinity if he not become a sharer in our mortality. That we have been made sharers in his divinity is said in the Gospel: “He gave them power to become children of God, to those who believe in his name, who were born not from blood, not from the will of the flesh, not from the will of a man, but of God.” And that this could happen because he himself was made a sharer in our mortality, it then follows: “And the Word was made flesh and dwelt among us” (Jn 1:12-14). By this participation grace is given to us that we may have a chaste fear of God and may keep his commandments.
So it’s Jesus himself who is speaking in this prophecy: he says some things in his members and in the unity of his Body as in a single person spread throughout the world and growing as the ages roll by; and he says some things in himself, our Head. That’s what he was doing when he said: “I am a sharer with all those who fear you and keep your commandments.” And because he became a sharer with his brothers, God with men and women, the immortal one with mortals, the grain fell to the ground so that it might die and bear much fruit. And he went on to say of this fruit: “The earth is full of your mercy, Lord.” And where else if not when a wicked person is made righteous? That he might grow in the knowledge of that grace, the Psalmist adds, “And teach me the ways of your righteousness.” (EnPs 118/16; PL 37, 1546-1547)