They must have strayed from something. What?
In the comments to the church at Ephesus God says "You hate the practices of the Nicolaitans, which I also hate". That word means "lay conquerors". I would suggest as many do that was referring to those who were trying to impose a hierarchy over the lay people. That's exactly what the Catholic Church has been doing for thousands of years.
That's an odd speculation, since Jesus instituted the church ("if he will not listen to the church, treat him as a pagan or tax collector") and the office of the bishop (see Acts, "Let his office [bishoprick in the KJV] another man take.")
This is a more likely explanation of the "sin of the Nicolaitans."
Around the year 110, about the same time that John was writing the Book of Revelation, St. Ignatius said the following:
The Epistle of Ignatius to the Smyrnaeans (110 A.D.)"If he will not listen to the church, treat him as a pagan or tax collector." --JesusChapter 7. Let us stand aloof from such heretics
They abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again. Those, therefore, who speak against this gift of God, incur death in the midst of their disputes. But it were better for them to treat it with respect, that they also might rise again. It is fitting, therefore, that you should keep aloof from such persons, and not to speak of them either in private or in public, but to give heed to the prophets, and above all, to the Gospel, in which the passion [of Christ] has been revealed to us, and the resurrection has been fully proved. But avoid all divisions, as the beginning of evils.
Chapter 8. Let nothing be done without the bishop
See that you all follow the bishop, even as Jesus Christ does the Father, and the presbytery as you would the apostles; and reverence the deacons, as being the institution of God. Let no man do anything connected with the Church without the bishop. Let that be deemed a proper Eucharist, which is [administered] either by the bishop, or by one to whom he has entrusted it. Wherever the bishop shall appear, there let the multitude [of the people] also be; even as, wherever Jesus Christ is, there is the Catholic Church. It is not lawful without the bishop either to baptize or to celebrate a love-feast; but whatsoever he shall approve of, that is also pleasing to God, so that everything that is done may be secure and valid. Chapter
9. Honour the bishop
Moreover, it is in accordance with reason that we should return to soberness [of conduct], and, while yet we have opportunity, exercise repentance towards God. It is well to reverence both God and the bishop. He who honours the bishop has been honoured by God; he who does anything without the knowledge of the bishop, does [in reality] serve the devil. Let all things, then, abound to you through grace, for you are worthy. You have refreshed me in all things, and Jesus Christ [shall refresh] you. You have loved me when absent as well as when present. May God recompense you, for whose sake, while you endure all things, you shall attain unto Him.
John was given MESSAGES to deliver to the seven CATHOLIC churches in Asia...
15 But if thy brother shall offend against thee, go, and rebuke him between thee and him alone. If he shall hear thee, thou shalt gain thy brother.
16 And if he will not hear thee, take with thee one or two more: that in the mouth of two or three witnesses every word may stand.
17 And if he will not hear them: tell the church. And if he will not hear the church, let him be to thee as the heathen and publican.