As one who is in the Lord Jesus, I am fully convinced that no food is unclean in itself.
1 Corinthians 10:25-27 (also 1 Cor 8:4-8)
Eat anything sold in the meat market without raising questions of conscience, for, "The earth is the Lord's, and everything in it."
If some unbeliever invites you to a meal and you want to go, eat whatever is put before you without raising questions of conscience.
You are to abstain from food sacrificed to idols, from blood, from the meat of strangled animals and from sexual immorality.
1 Corinthians 10:20-21
...the sacrifices of pagans are offered to demons, not to God, and I do not want you to be participants with demons. You cannot drink the cup of the Lord and the cup of demons too; you cannot have a part in both the Lord's table and the table of demons.
Nevertheless, I have this against you: You tolerate that woman Jezebel, who calls herself a prophetess. By her teaching she misleads my servants into sexual immorality and the eating of food sacrificed to idols.
Paul's teaching, which he explains in Romans 14:14-23 and 1 Corinthians 8:4-13, is that eating food sacrificed to idols is not wrong in and of itself, but is better avoided for the sake of Christians who think it is wrong and would consequently be led into sin (Rom 14:23).
While eating food that has been sacrificed to idols is not wrong, participating in idol worship is. Part of the idol worship of the time may have involved sacrificing meat to an idol and then feasting on the meat in celebration of the idol. In this case, participating in the feast would be participating in the worship of the idol, just as partaking of Communion is an act of worshiping Christ - a parallel Paul draws in 1 Corinthians 10:15-18. Therefore vv. 20-21 refer to eating sacrificed meat at an idol feast and thus practicing idolatry (as opposed to buying sacrificed meat in the market, or eating sacrificed meat at an ordinary meal in someone's home).
The condemnation of eating sacrificed meat in Acts 15:29 and Revelation 2:20 probably refers to participation in an idol feast, but could also refer to the sin of leading others into sin by eating it (1 Cor 8:10-12).