For context on this amazing quotation, see one of my favorite blogs Roger Pearse.
And should you hear any one in the public thoroughfare, or in the midst of the forum, blaspheming God; go up to him and rebuke him; and should it be necessary to inflict blows, spare not to do so.
Smite him on the face; strike his mouth; sanctify thy hand with the blow, and if any should accuse thee, and drag thee to the place of justice, follow them thither; and when the judge on the bench calls thee to account, say boldly that the man blasphemed the King of angels!
For if it be necessary to punish those who blaspheme an earthly king, much more so those who insult God. It is a common crime, a public injury; and it is lawful for every one who is willing, to bring forward an accusation.
Let the Jews and Greeks learn, that the Christians are the saviours of the city; that they are its guardians, its patrons, and its teachers.
Let the dissolute and the perverse also learn this; that they must fear the servants of God too; that if at any time they are inclined to utter such a thing, they may look round every way at each other, and tremble even at their own shadows, anxious lest perchance a Christian, having heard what they said, should spring upon them and sharply chastise them.
- St. John Chrysostom