The following interpretation, of the book of Revelation, on the Nicolaitans, by Merrill Chapin Tenney is the best one I’ve been able to find so far:
‘The condemnation of the church at Pergamum betrays a moral laxity which had crept into it. “The teaching of Balaam” recalls the Old Testament episode of Balaam’s attempt to serve God and at the same time to please his enemies.  Summoned by Balak, king of Moab, to curse the people of God, he refused because God would not give him permission to do so. When Balak offered him richer rewards, he tried again to see whether God would change His mind. God allowed him to go to Moab, but compelled him to utter blessings instead of curses, with the result that Balaam was embarassed and that Balak was exasperated. Finally when he could not curse Israel, he taught Balak how to corrupt them so that they brought the judgment of God upon themselves by their immorality and idolatry. Balaam represents those who foster evil and disobedience to God under a guise of piety.
Evidently this error of Balaam was the chief principle of the sect of the Nicolaitans. Aside from the record in Revelation, little is known of them. Irenaeus, who wrote in the second century, nearly a hundred years after Revelation, said that they were founded by Nicolaus the proselyte of Antioch mentioned in Acts 6:5, and that they “lived lives of unrestrained indulgence.”  Irenaeus attributed to them certain doctrinal vagaries which are not mentioned in the Apocalypse. It is possible that if the sect survived until this day it might have developed new peculiarities; but the main errors stressed here are eating things sacrificed to idols and commiting fornication. These carry out the practices which entered Israel because of Balaam, and illustrate the peril which Paul sought to avert by his warnings to the church at Corinth (I Cor. 10:7,8). In a civilization where temples to false gods stood on every street corner and in every public square, in which sexual indulgence was not only condoned but was in many of the cults promoted as an act of worship, the Christians who had been brought up in that environment had a hard time to break away from it completely. The teaching of the Nicolaitans was an exaggeration of the doctrine of Christian liberty which attempted an ethical compromise with heathenism.
Whereas in the Ephesian church the Nicolaitans were known by their works, in Pergamum they were known by their teaching. Sporadic acts had become settled doctrine; and it is possible that the same group was represented in Thyatira by “the woman Jezebel, who… seduceth my servants to commit fornication, and to eat things sacrificed to idols” (Rev. 2:20). In Ephesus they are outside the church, known by their works; in Pergamum they are still an alien sect, but a few hold their teachings; in Thyatira their doctrines are propagated by a person within the group who is at least tolerated as one of them.
This moral decline of the church is sharply rebuked by the Lord who is keenly aware of the peril. His correction, “Repent therefore; or else I come to thee quickly, and I will make war against them with the sword of my mouth,” threatens swift matching of His spiritual authority with false ecclesiastical authority, and the consequent purging of the church. The sword of His mouth is mentioned again in Revelation 19:15 as the means of His judgment upon the unbelieving nations at large? It is His word of truth put into action, the direct application to divine righteousness. The judgment of the world at large will be visited upon the churches if they do not sever themselves from its evils by repentance.’
 See Num. 22:1-25:9; II Pet. 2:15, 16.
 Irenaeus, Against Heresies, I, xxvi, 3; III, xi, 1.
Source: Merrill C. Tenney, Interpreting Revelation, Vision I: The Seven Churches of Asia (1957), p 60-61 (all emphases are his).
All Scripture passages taken from the American Standard Version of the Bible, copyrighted 1929 by the Division of Christian Education, National Council of Churches, and are used by permission.
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