2:15 `Thus you also have some who in the same way hold the teaching of the Nicolaitans.
Thus you also have some who in the same way hold the teachings of the Nicolaitans.
The Ephesus church (Revelation 2:6) discovered the deeds of the Nicolaitans and didn't tolerate them.
The Pergamum church had them in the congregation expousing their cultic teachings. It is not known who these Nicolaitans were but God certainly hated them.
It is interesting to note that Satan tried to put pressure on the churches of Smyrna and Pergamum from the outside through persecution and martyrdom respectively. He didn't succeed.
How easy it is to criticize "outside" faults of churches when, in fact, the devil may be inside already. Pastors (shepherds) have an awesome responsibility to keep the "wolves out of the sheep".