Surah 33. Al-Ahzab, Ayah 58

 By harming them physically or accusing them falsely.

Cf. iv. 112. In that passage we were told that any one who was himself guilty but accused an innocent man of his guilt, was obviously placing himself in double jeopardy; first, for his own original guilt, and secondly for the guilt of a false accusation. Here we take two classes of men instead of two individuals. The men and women of faith (if they deserve the name) and doing all they can to serve Allah and humanity. If they are insulted, hurt, or annoyed by those whose sins they denounce, the latter suffer the penalties of a double guilt, viz., their sins to start with, and the insults or injuries they offer to those who correct them. Instead of resenting the preaching of Truth, they should welcome it and profit by it.