When Mary told the angel, “Be it done to me according to your word,” God, who was present as God on earth from the dawn of creation, began to be present on earth as the God-man. In other words, God began to exist as man the moment Our Lady consented to becoming the Mother of her Creator.
Once God became man, He intended to remain Incarnate for all eternity. He lived a human mortal life until His death on the cross. When He rose from the dead, He remained on earth visibly for forty days. Then He ascended into heaven and, as we profess in the Creed, He is seated at the right hand of His heavenly Father.
But on the night before He died, He ordained His apostles as priests, mainly to continue doing what He had done when He changed bread and wine into His own living Body and Blood. Why did He do this? In order to be not only in heaven, but to remain on earth as the God-man in the Holy Eucharist until the end of time.
It is impossible to exaggerate the role of Mary in the Holy Eucharist. Except for her, there would not have been any Incarnation, and without the Incarnation there could be no Eucharist.
Christ in the Eucharist, therefore, is identical with the Christ of history in possessing the same human nature. But this presents two more possibilities. Having the same human nature can mean that in both cases He has only the same human soul, or that He also has the same human body.
Orthodox Catholic doctrine teaches, of course, that the identity of Christ’s human nature covers both body and soul. Consequently we find a stress in the documents of the Church on a bodily presence in the Blessed Sacrament, as a counterpoise to the error of spiritualizing Christ in the Eucharist to the exclusion or at least the oversight of His body. The Church’s infallible teaching could not be clearer. In the Eucharist is present the whole Christ, humanity and divinity, body and soul. Everything which makes Christ, Christ is present in the Blessed Sacrament.
As Jesus told the crowd, “Amen, Amen, I say to you, unless you eat the flesh of the Son of Man, and drink His blood you shall not have life in you” (John 6:54).
As Catholics we believe that Jesus Christ is present on earth in the fullness of His humanity. Unless this were true, there could be no adoration of the Blessed Sacrament. There could be no Sacrifice of the Mass, and no Holy Communion.
All of this we owe to the Blessed Virgin Mary once she agreed to conceive the Incarnate Son of God and give Him birth at Bethlehem. (Source)