One of the common claims made by the Protestants is that the Catholic teaching on Christ's real Body and Blood being present after the consecration in the Mass is a late invention of a heretical Roman church. Yet, it is not hard to find many, many examples of the Catholic teaching in the early writings of the Church. Saint John Damascene, often regarded as the last of the Eastern Fathers wrote a wonderful exposition on the basics of the Christian faith, presented in four parts. As I was reading through Book IV, I came across his thoughts on the presence of Christ in the Eucharist. Who would have thought that Saint John, writing in the early 700s would be teaching Catholic doctrine? Of course we know that it is the case, but for your enrichment I wanted to provide some of the quotes from Book IV of his work. It is clear that Saint John did not have as deep of an explanation as to how Christ was able to be present in the Eucharist as Saint Thomas and the scholastics did later. But there is no question that he did indeed believe that He was present after the consecration in His entire person. In fact he says, "...that the bread itself and the wine are changed into God's body and blood. But if you enquire how this happens, it is enough for you to learn that it was through the Holy Spirit, just as the Lord took on Himself flesh that subsisted in Him and was born of the holy Mother of God through the Spirit."
Chapter XIII covers the 'Holy and Immaculate Mysteries of the Lord'. Here there is no ambiguity in his teaching as to how Christ is bodily present in the Sacrament. In fact he tells us,
The bread and the wine are not merely figures of the body and blood of Christ (God forbid!) but the deified body of the Lord itself: for the Lord has said, This is My body, not, this is a figure of My body: and My blood, not, a figure of My blood. And on a previous occasion He had said to the Jews, Except you eat the flesh of the Son of Man and drink His blood, you have no life in you. For My flesh is meat indeed and My blood is drink indeed. And again, He that eats Me, shall live John 6:51-55. Wherefore with all fear and a pure conscience and certain faith let us draw near and it will assuredly be to us as we believe, doubting nothing. Let us pay homage to it in all purity both of soul and body: for it is twofold.Saint John then proceeds to write about the grace of deification that is communicated through the Sacrament, signifying that not only is it the very Body and Blood of the Savior present, but that God actually deifies and changes man by his receiving Him in the Sacrament.
Let us draw near to it with an ardent desire, and with our hands held in the form of the cross let us receive the body of the Crucified One: and let us apply our eyes and lips and brows and partake of the divine coal, in order that the fire of the longing, that is in us, with the additional heat derived from the coal may utterly consume our sins and illumine our hearts, and that we may be inflamed and deified by the participation in the divine fire.Furthermore Saint John speaks of their being an actual consecration by a priest, another reality that the Protestants will not acknowledge. Where is the consecration in a "Reformed" service? Let me give you a hint, there isn't one. Another glaring reality that the "Reformed" sects are not part of the same Church that Saint John was a part of.
... just as in nature the bread by the eating and the wine and the water by the drinking are changed into the body and blood of the eater and drinker, and do not become a different body from the former one, so the bread of the table and the wine and water are supernaturally changed by the invocation and presence of the Holy Spirit into the body and blood of Christ, and are not two but one and the same...But if some persons called the bread and the wine antitypes of the body and blood of the Lord, as did the divinely inspired Basil, they said so not after the consecration but before the consecration, so calling the offering itself.Finally like the Catholic Church teaches today, Saint John also taught that those heretics who reside outside the Church should never be granted the privilege of receiving the Body and Blood of Christ in the Sacrament.
With all our strength, therefore, let us beware lest we receive communion from or grant it to heretics; Give not that which is holy unto the dogs, says the Lord, neither cast ye your pearls before swine Matthew 7:6, lest we become partakers in their dishonour and condemnation. For if union is in truth with Christ and with one another, we are assuredly voluntarily united also with all those who partake with us. For this union is effected voluntarily and not against our inclination. For we are all one body because we partake of the one bread, as the divine Apostle says 1 Corinthians 10:17 .