"Jesus said to them,
amen, I say to you,
unless you eat the flesh of the Son of Man
and drink his blood,
you do not have life within you." (Jn 6:55)
We are told by St. John, many of his disciples left Jesus because they did not understand how this could be possible. Do you have difficulty with this truth of faith? Perhaps this difficulty arises because you think of Christ body in biological terms which he surely had as man but now Christ resides in a dimension of reality invisible to us.
Events after the Resurrection make us aware of this. For example, He appeared to the Apostles in the upper room by passing through closed doors yet he told Thomas to touch him since it was Himself. The apostles saw Jesus walking on water after the resurrection, yet to reassure them that he was not a ghost by eating with them. Somehow Christ revealed his presence in a tangible form although his body was now transfigured.
When the priest breaks the host, does he cut Christ in two? When you eat his body, do you break his body up? No at all because in each particle of bread and drop of wine, He is tautly present in body-soul and divinity. He remains unharmed by all these actions and can be present simultaneously in different places. It is like saying that in every drop of water the whole quantity of ocean water is contained.
Thomas Aquinas tells us that the reality of the consecrated bread is unlike that of nature.
In nature we see that the changes in external appearance (e.g. a tree appears different in the fall than it does in winter) but its nature does not change, it still remainsa tree).
The consecrated bread instead, appears as bread externally but internally it is no longer bread but the "Body of Christ". Invisibly, the bread is no longer bread but the living living Christ in his humanity and divinity.
In the Middle Ages some maintained that after the Consecration bread looked like bread and was still bread in its nature or substance. To this St. Thomas replies,