Acts 18:24-26 (Appollos); Revelations 2:12-16 (Nicolatians)
John identifies the teaching of Balaam with two problems: "eating food sacrificed to idols" and "sexual immorality".
The early church constantly struggled with compromises with paganism, as we see in Paul's long discussion in 1 Corinthians 8 - 10, as well as in the conclusions reached in Acts 15:20, 29. Both of these center on food offered to idols, Paul's conclusion being that one could eat such food if purchased in the marketplace, but one should not go to a meal in a pagan temple.
Following this Pauline rule, however, would cut one off from membership in trade guilds, patriotic celebrations (including ceremonies honoring the emperor, considered essential to good citizenship) and many family celebrations. We can easily see the pressure to rationalize and thereby develop a compromise.
The issue of sexual immorality is more difficult, for it is also mentioned in Revelation 2:20, 22, in the case of Jezebel (an Old Testament code word for a New Testament woman leader of the church in Thyatira, indicating her spirit and God's evaluation, rather than the woman's actual name). On the one hand, sexual immorality was a problem in the early church, as Paul's discussions show (1 Corinthians 5:1; 6:12-20; compare Hebrews 13:4).
In the middle of a pagan society that accepted the use of prostitutes (although wives were expected to remain faithful), it was difficult to remain obedient on this point and relatively easy to compromise.
On the other hand, "sexual immorality" was used in the Old Testament for involvement with pagan deities. For example, the Old Testament Jezebel was not to our knowledge physically immoral - she was likely faithful to Ahab all her life - but she did lead Israel into Baal worship. Since Israel was Gods' "bride", such involvement with other gods was called "adultery" or "sexual immorality".
If then, John is taking the Old Testament examples as the basis for his discussion; the sexual immorality is figurative, standing for their worship of other deities, which was implied in their attending feasts in idol temples.
The Nicolatians, then, appear to be a group that corrupted Gods' people by suggesting compromise with the culture of the day. Whie the exact issues are different, similar compromises face the church today. Each society has its own "idols" that it expects all its citizens to worship, whether those idols be the government itself or some values or practices of the society. Furthermore, the Nicolatians are still with us under a variety of names, for there are always people who in the name of being "realistic" or under any number of other theological justifications counsel compromise with the dominant culture. This passage warns us that Jesus will not "buy" these justifications. He demands nothing less than total loyalty to his own person and directions. Anything less than this will put those who compromise in danger of his judgement.