From a sermon by Saint Leo the Great, pope

My dear brethren, there is no doubt that the Son of God took our human nature into so close a union with himself that one and the same Christ is present, not only in the firstborn of all creation, but in all his saints as well. The head cannot be separated from the members, nor the members from the head. Not in this life, it is true, but only in eternity will God be all in all, yet even now he dwells, whole and undivided, in his temple the Church. Such was his promise to us when he said: See, I am with you always, even to the end of the world.

And so all that the Son of God did and taught for the world’s reconciliation is not for us simply a matter of past history. Here and now we experience his power at work among us. Born of a virgin mother by the action of the Holy Spirit, Christ keeps his Church spotless and makes her fruitful by the inspiration of the same Spirit. In baptismal regeneration she brings forth children for God beyond all numbering. These are the sons of whom it is written: They are born not of blood, nor of the desire of the flesh, nor of the will of man, but of God.

In Christ Abraham’s posterity is blessed, because in him the whole world receives the adoption of sons, and in him the patriarch becomes the father of all nations through the birth, not from human stock but by faith, of the descendants that were promised to him. From every nation on earth, without exception, Christ forms a single flock of those he has sanctified, daily fulfilling the promise he once made: I have other sheep, not of this fold, whom it is also ordained that I shall lead; and there shall be one flock and one shepherd.

Although it was primarily to Peter that he said: Feed my sheep, yet the one Lord guides all the pastors in the discharge of their office and leads to rich and fertile pastures all those who come to the rock. There is no counting the sheep who are nourished with his abundant love, and who are prepared to lay down their lives for the sake of the good shepherd who died for them.

But it is not only the martyrs who share in his passion by their glorious courage; the same is true, by faith, of all who are reborn through baptism. That is why we are to celebrate the Lord’s paschal sacrifice with the unleavened bread of sincerity and truth. The leaven of our former malice is thrown out, and a new creature is filled and inebriated with the Lord himself. For the effect of our sharing in the body and blood of Christ is to change us into what we receive. As we have died with him, and have been buried and raised to life with him, so we bear him within us, both in body and in spirit, in everything we do.

Via Divine Office.org