Revelation Chapter 2
Verse 14 (NASB) 
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14. `But I have a few things against you, because you have there some who hold the teaching of Balaam, who kept teaching Balak to put a stumbling block before the sons of Israel, to eat things sacrificed to idols, and to commit acts of immorality.
BUT I have a few things against you, because you have
some who hold the teaching of BALAAM, who kept teaching
BALAK to put a stumbling block before the SONS of ISRAEL,
(1) to eat things sacrificed to idols, andWhy should this be so bad? If one could eat things sacrificed to idols this may indicate that one tolerates (or even worships) these idols. God is a very jealous God. He will not have ANY god before Him - Exodus 20:3.
The teachings of Balaam was that it was alright to intermarry with heathen and compromise over idolatrous worship. One can see this in Numbers chapters 22 through 25.
(2) to commit acts of immorality.At the time of Revelation's writing, men indulged in sexual intercourse with temple priestesses in cult prostitution. Even if this phrase didn't refer to temple prostitution but rather to spiritual unfaithfulness, this, too, is not right in God's sight. See Ezekiel 23:36-39. It is so easy to follow the advise of "friends" or because "they are doing it..." makes the immorality OK. Not so. John tells us earlier in his life, 1 John 5:18-21, that we are to guard ourselves from idols.
References to the Revelation of Jesus Christ are in bold italics. All other references are in bold only.