To those who have doubts or uncertainty about the transubstantiation of the bread into the Body and the wine into the Blood of our Lord Jesus Christ there are many ways of explaining.
1. Every Orthodox Christian should ask those who have the above doubt this question: Can God do more than man, and above his understanding?… Yes? Then why cannot He give us His Flesh as food? Just as by the word of the Lord the heavens were established (Psalm 36, 6), so by the Word of God the bread and wine are changed into His Body and Blood.
2. If anyone wonders how the same Christ can be on the altar and in heaven, let him wonder also how the one sun, which lights and warms us, is at the same time on earth and in the sky, in East and West, and in all the countries of the world. So Christ, the Almighty, can be at once in heaven and in the Holy Mysteries, in heaven by true substance and on earth by the power of His incomprehensible and ineffable Divinity, beyond the mind of man, performing His great and glorious works.
3. And again, if he wonders how one whole Christ is given to the faithful equally in many parts, not less to one nor more to the others, let him wonder also at this, how my one voice is in my mouth and in your ears, and at the same time is one voice.
4. If he wonders how the Body in the Sacrament is not broken when Lamb is divided, or how in every part is the whole Christ, let him wonder also at this, when a mirror is broken, but in each part it appears whole, as in the full mirror.
5. If he wonders how Christ Whom we so often eat does not decrease but remains whole forever, let him wonder also at this, when you light from one candle other candles, the light of the candle does not diminish.
6. If you ask how Christ Who comes into our nature is not defiled or confined, I will ask you: When the sun passes through a foul-smelling place, is it defiled? Far more, Christ, the light of all purity, is not defiled or confined, He Whom hell could not hold, nor the sealed Tomb, and Who could pass through to the disciples when the doors were firmly closed.
7. The awful Mysteries which we eat do not pass out of the body with our ordinary food, but as St. Chrysostom says the consecrated bread and wine is changed into a kind of fine vapour which penetrates into all the parts of our body. Likewise, if the Holy Mysteries happen through carelessness or chance to fall, corrupt, be burnt, be frozen, fall into dirt, etc., it is not the Body of Christ which suffers but the outward appearance of the Bread and Wine. The painless Sacrifice has no part in these things — impossible! Only the Body of Christ suffered for us, and after the Resurrection, it cannot suffer. But the priest is not less blameworthy for his carelessness — grave are his sins, and great must be the punishment and penance according to the judgment of his bishop.
8. Do you wonder how in the little particles of the Sacrament can be the whole Christ? Then wonder how in the pupil of the eye such big cities can be contained. But do not try to probe into this Mystery, but with undoubting faith and love, with heart and deeds give thanks to Almighty God for His unspeakable gifts.
9. How are we to prepare to receive the Body and Blood of Christ? Ask the blessed Paul, he will teach you two useful virtues, first: Let a man examine himself… (1, Cor. 9, 28). Second, let him discern what the Body and Blood of the Lord is, who you are and how are, to Whom you are to be united, and how you are approaching, and with Whom you have Communion.
10. See that you have a pure heart, are at peace with God, of one mind with His Church, and have love towards your neighbour. As long as we remain in sin, we have not peace with God, for our sins separate us from God, according to Isaiah 69, 2 and make us enemies of God, and give joy and gladness to the devil.
11. If you remain in heresy, and do not correct yourself, and remain obstinate, and if you do not believe as the Church of Christ and the Holy Spirit and the Word of God teach us who are the sons of the Church, you are not at peace with Christ and you cannot be worthy to partake of the Body and Blood of Christ, which is union and reconciliation with God.
12. You must believe with the Church about this awful Mystery. Though you see with your bodily eyes visible bread and wine, yet believe firmly without doubt that the substance of them by the coming and action of the Holy Spirit, and by the Power of the Almighty Word of God, is changed into the Body and Blood of Christ, and that it is no other than the true Body and Blood of Christ under the appearance of fresh, soft, leavened, wheat bread, and wine made from a bunch of grapes by pressing.
Source: Orthodox Life 1950 Vol 3, Holy Trinity Monastery, Jordanville