Adam Clarke on Revelation 2:6

The deeds of the Nicolaitanes - These were, as is commonly supposed, a sect of the Gnostics, who taught the most impure doctrines, and followed the most impure practices. They are also supposed to have derived their origin from Nicolas, one of the seven deacons mentioned Act 6:5 (note). The Nicolaitanes taught the community of wives, that adultery and fornication were things indifferent, that eating meats offered to idols was quite lawful; and mixed several pagan rites with the Christian ceremonies. Augustine, Irenaeus, Clemens Alexandrinus, and Tertullian, have spoken largely concerning them. See more in my preface to 2d Peter, where are several particulars concerning these heretics.