cyril_alexandriaHe that eats My Flesh and drinks My Blood dwells in Me and I in him (John 6:56). 

[…] If one should join wax with other wax, he will surely see…the one in the other.

In like manner…he who receives the Flesh of our Saviour Christ and drinks His Precious Blood, as He says, is found one with Him.

He is commingled as it were and immingled with Him through the participation, so that he is found in Christ, and Christ again in him.

Thus was Christ teaching us in the Gospel too according to Matthew, saying, The Kingdom of heaven is like unto leaven, which a woman took and hid in three measures of meal, till the whole was leavened. 

[…] As then Paul says that a little leaven leaves the whole lump, so the least portion of the Blessing [i.e. the Sacrament] blends our whole body with itself, and fills it with its own mighty working, and so Christ comes to be in us, and we again in Him.

For one may truly say that the leaven is in the whole lump, and the lump by like reasoning is in the whole leaven: you have in brief the sense of the words.

And if we long for eternal life, if we pray to have the Giver of immortality in ourselves, let us not like some of the more heedless refuse to be blessed, nor let the devil, deep in wickedness, lay for us a trap and snare a perilous reverence….

For it is written, He that eats of the Bread, and drinks of the Cup unworthily, eats and drinks doom unto himself. 

And I, having examined myself, see that I am not worthy.

When then will you be worthy…when will you present yourself to Christ?

For if you are always going to be scared away by your stumblings, you will never cease from stumbling…, and will be found wholly without participation of that wholly-preserving sanctification.

Decide then to lead a holier life, in harmony with the law, and so receive the Blessing, believing that it has power to expel, not death only, but the diseases in us.

For Christ, coming to be in us in this way, lulls the law which rages in the members of the flesh, and kindles piety towards God, and deadens our passions, not imputing to us the transgressions in which we are, but rather, healing us, as sick.

For He binds up that which was crushed, He raises what had fallen, as a Good Shepherd and One that hath laid down His Life for His sheep.

Cyril of Alexandria (c. 376-444): Commentary on St John’s Gospel, book 4, c.3 [on John 6:56].