“I speak not only to the communicant, but also I say to the priest who ministers the Sacrament: Distribute this gift with much care. There is no small punishment for you, if being conscious of any wickedness in any man, you allow him to partake of the banquet of the table: ‘Shall I not now require his blood at your hand?’ (2 Sam. 4:11). If some public figure, or some wealthy person who is unworthy, presents himself to receive Holy Communion, forbid him. The authority that you have is greater than his. Consider if your task were to guard a clean spring of water for a flock, and you saw a sheep approach with mire on its mouth–you would not allow it to stoop down and pollute the stream. You are now entrusted with a spring, not of water, but of blood and of spirit. If you see someone having sin in his heart (which is far more grievous than earth and mire), coming to receive the Eucharist, are you not concerned? Do you try to prevent him? What excuse can you have, if you do not?
“God has honored you with the dignity of priesthood, that you might discern these things. This is not to say that you should go about clothed in a white and shining vestment; but this is your office; this, your safety; this your whole crown.
“You ask how you should know which individual is unworthy to receive? I am speaking here not of some unknown sinner, but of a notorious one. If someone who is not a disciple, through ignorance, comes to Communion, do not be afraid to forbid him. Fear God, not man. If you fear man, you will be scorned and laughed at even by him; but if you fear God, you will be an object of respect even to men. But if you cannot do it, bring that sinner to me, for I will not allow anyone to dare do these things. I would give up my life rather than give the Lord’s Blood to the unworthy.
“If, however, a sinful person receives Communion, and you did not know his character, you are not to blame, however. I say the things above concerning only those who sin openly. For if we amend these, God will speedily reveal to us the unknown also; but if we let these flagrant abuses continue, how can we expect Him to make manifest those that are hidden? I say these things, not to repel sinners or cut them off, but I say it in order that we may bring them to repentance, and bring them back, so that we may take care of them. For thus we shall both please God and lead many to receive worthily. And for our own diligence, and for our care for others, we will receive a great reward. May we attain that reward by the grace and love that God gives to man through Our Lord Jesus Christ, to whom be glory, world without end. Amen.”
St. John Chrysostom, Sermon: On the Institution of the Eucharist