Adam Clarke's Commentary on the Bible

Verse 21 (Revelation 9:21)

Neither repented they of their murders - Their cruelties towards the genuine followers of God, the Albigenses, and Waldenses, and others, against whom they published crusades, and hunted them down, and butchered them in the most shocking manner. The innumerable murders by the horrible inquisition need not be mentioned.

Their sorceries - Those who apply this also to the Romish Church understand by it the various tricks, sleights of hand, or legerdemain, by which they impose on the common people in causing images of Christ to bleed, and the various pretended miracles wrought at the tombs, etc., of pretended saints, holy wells, and such like.

Fornication - Giving that honor to various creatures which is due only to the Creator.

Their thefts - Their exactions and impositions on men for indulgences, pardons, etc. These things may be intended, but it is going too far to say that this is the true interpretation. And yet to express any doubt on this subject is with some little else than heresy. If such men can see these things so clearly in such obscure prophecies, let them be thankful for their sight, and indulgent to those who still sit in darkness.

- Adam Clarke's Commentary on the Bible