It is not surprising that our Redeemer should have chosen the blessed John to expound in his stead the mystery of his divinity, for he also deputed John to replace him as Mary's son: he was to have the great and ever-virgin mother in his keeping on his Lord's behalf—Woman, this is thy son, came the words, and to the disciple, This is thy mother.2 Christ is the Mediator between God and men3 because he unites the natures of them both, the divine nature and the human, in the person of the one Emmanuel. Thus, it was highly fitting that the blessed John should give his faithful service to each of those natures, on the one hand committing the mysteries of the divine nature to writing, and on the other devoting his constant attention to the welfare of the blessed mother.
The Lord entrusted the keys of the Church to Peter and he chose to give the custody of Mary to John. Now these two are both mothers: Mary is a mother and the Church is a mother. Mary is Christ's Mother, the Church is the mother of Christ's people. Mary gave Christ his flesh; and out of his flesh, the flesh of his side, Christ brought the Church to birth. He came forth from Mary when he was born in the flesh; he brought forth the Church when he died in the flesh. He chose to be born of the one, he deigned to die for the other. Of the one he was born in his own person, once and for all; of the other his members are born day after day. From the one he drew the means of dying for the other and the ground of their common salvation. Great and favored in her motherhood was the blessed Virgin Mary, for her body supplied the flesh of Christ, from which again the Church flowed forth on a tide of blood and water; so that in this respect the Church may be regarded as proceeding from Mary as well. And both Mary and the Church are chaste and pure; both are protected by the girdle of perpetual maidenhood.
Of these two mothers, then, these two perpetual virgins, the Lord chose to commit one to Peter and the other to John, because he wanted us to consider carefully the services rendered by both those members of the court of heaven and estimate their value, and so to see that for excellence and worth and merit each was the other's equal. The Lord said to Peter: I will give to thee the keys of the kingdom of heaven. But the blessed Virgin, brethren: was not she a heaven too, if for nine whole months the Godhead in all its fullness rested in her womb? From her clean, chaste body came the flesh that clothed the Word of God, through whom all things came into being. 2 There he lay, cramped in the narrow confines of her womb, who in his boundless power defines the limits of all human laws. His dwelling a scant space in a virgin body, the heavens were still at his command and he held undiminished sway over the things of earth. So it is not too much to give the name of heaven to the object of our veneration, the Virgin Mary and therefore, when the Lord appointed the blessed John as his mother's guardian, he gave him too, in a way, the keys of the kingdom.