John of Damascus on the Holy Supper (Or: Why Martin Chemnitz Liked Him So Much)

If then the Word of God is quick and energising, and the Lord did all that He willed; if He said, Let there be light and there was light, let there be a firmament and there was a firmament; if the heavens were established by the Word of the Lord and all the host of them by the breath of His mouth; if the heaven and the earth, water and fire and air and the whole glory of these, and, in sooth, this most noble creature, man, were perfected by the Word of the Lord; if God the Word of His own will became man and the pure and undefiled blood of the holy and ever-virginal One made His flesh without the aid of seed, can He not then make the bread His body and the wine and water His blood? …
Wherefore to those who partake worthily with faith, it is for the remission of sins and for life everlasting and for the safe-guarding of soul and body; but to those who partake unworthily without faith, it is for chastisement and punishment, just as also the death of the Lord became to those who believe life and incorruption for the enjoyment of eternal blessedness, while to those who do not believe and to the murderers of the Lord it is for everlasting chastisement and punishment.
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. …
Participation is spoken of; for through it we partake of the divinity of Jesus. Communion, too, is spoken of, and it is an actual communion, because through it we have communion with Christ and share in His flesh and His divinity: yea, we have communion and are united with one another through it. For since we partake of one bread, we all become one body of Christ and one blood, and members one of another, being of one body with Christ.
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, saith the Lord, neither cast ye your pearls before swine, 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. (John of Damascus, Exposition of the Orthodox Faith, Ch. XIII)

[Timotheos]