Followers of Nicolaus, a heretic.
They are thought to have been a sect of Gnostics who practiced and taught impure and immoral doctrines
such as the community of wives,
that committing adultery and fornication was not sinful,
and that eating meats offered to idols was lawful.
This was similar to the doctrines of Balaam and Jezebel
The Nicolaitans are mentioned in two of the letters addressed to the seven churches in Rev 2–3:
Revelation 2:6 (NKJV) But this you have, that you hate the deeds of the Nicolaitans, which I also hate.
Rev 2:6 affirms the believers at Ephesus because they “hate the deeds of the Nicolaitans.” This verse offers no further details that might point to the Nicolaitans’ identity or teachings.
Revelation 2:15 (NKJV) Thus you also have those who hold the doctrine of the Nicolaitans, which thing I hate.
Rev 2:15 calls for repentence from the believers at Pergamum, some of whom “hold fast to the teaching of the Nicolaitans.”
Here, an association with Balaam (Rev 2:14) connects the Nicolaitans with those who eat food sacrificed to idols and practice sexual immorality.
These same behaviors reflect Jezebel’s false teachings, which are described several verses later in the letter to Thyatira (Rev 2:20).