‘But I have a little against you, because you have there some who hold the teaching of Balaam, who taught Balak to cast a stumblingblock before the children of Israel, to eat things sacrificed to idols and to commit immorality. In the same way you also have some who hold the teaching of the Nicolaitans.'
The reference is to Numbers 25:3 onwards, compare 2 Peter 2:15; Jude 1:10-13. It is clear that these ideas were fairly widespread (compare Revelation 2:6). Balaam was a soothsayer and worker of magic, called in to bring down the power of the gods to destroy Israel. Yet under the influence of God he blessed them instead. His name (bala ‘am) probably means ‘he who swallows down the nation'. (Compare Nicolaus, ‘he who conquers the people'). And in spite of blessing the people of Israel he did them harm nevertheless.
Eating things sacrificed to Roma and the Emperor would have great significance as being looked on as a compromise with Emperor worship, giving it a kind of approval. (Later it would be demanded as proof of loyalty to Rome). It refers to any participation of Christians in marginal religions and beliefs which could give a false impression to outsiders.
Immorality, or involvement with sex for its own sake, has always been men's downfall. In contrast with the sex which seals the marriage bond, it is totally contrary to the teaching of Christ. Peter speaks very vividly of these people, ‘revelling in their love-feasts --- having eyes full of adultery, and who cannot cease from sin, enticing unsteadfast souls' (2 Peter 2:13-14).
The Nicolaitans held the same views, but were clearly not alone in them. Compromise with idolatry and sexual excesses were thus seen by Jesus as two of the greatest dangers to His people. (The word (oliga), literally translated ‘a few things' in versions, often means ‘a little' (e.g. Zechariah 1:15 LXX) and probably means that here). These things were the product of Babylon the Great.