Jamieson, Fausset and Brown on Revelation 9:21

21. sorceries--witchcrafts by means of drugs (so the Greek). One of the fruits of the unrenewed flesh: the sin of the heathen: about to be repeated by apostate Christians in the last days, Re 22:15, "sorcerers." The heathen who shall have rejected the proffered Gospel and clung to their fleshly lusts, and apostate Christians who shall have relapsed into the same shall share the same terrible judgments. The worship of images was established in the East in A.D. 842.

fornication--singular: whereas the other sins are in the plural. Other sins are perpetrated at intervals: those lacking purity of heart indulge in one perpetual fornication [Bengel].