Act 15:1-2 And certain men came down from Judea and taught the brethren, "Unless you are circumcised according to the custom of Moses, you cannot be saved." Therefore, when Paul and Barnabas had no small dissension and dispute with them, they determined that Paul and Barnabas and certain others of them should go up to Jerusalem, to the apostles and elders, about this question.
Paul after His miraculous conversion and the direct teaching and revelation from God while in Arabia realised that the Old Covenant was made obsolete by the New Covenant of grace, which was given to both Jews and Gentiles. The Gentile converts were enjoying the grace of God - without any law except for their repentance from idolatry and wickedness and their obedience to the teachings of Jesus.
But those so-called godly converts from Jews and Pharisees, were trying to impose Mosiac Law upon Christians, which Paul and Barnabas strongly opposed this heresy.
Act 15:5 But some of the sect of the Pharisees who believed rose up, saying, "It is necessary to circumcise them, and to command them to keep the law of Moses."
Doctrines of the Apostles
Act 15:28-29 For it seemed good to the Holy Spirit, and to us, to lay upon you no greater burden than these necessary things: that you abstain from things offered to idols, from blood, from things strangled, and from sexual immorality. If you keep yourselves from these, you will do well.
The council of Elders in Jerusalem under the guidance of the Holy Spirit confirmed that all Christians are to observe the doctrines of the apostles (teaching and commandments of Jesus) with the addition of the instructions given by God since the days of Noah. No Christian is allowed to observe the law which was made obsolete by the blood of Jesus in the New Covenant.
Evil Workers and Mutilation
Phi 3:2-3 Beware of dogs, beware of evil workers, beware of the mutilation! For we are the circumcision, who worship God in the Spirit, rejoice in Christ Jesus, and have no confidence in the flesh.
Paul realised that from within the church, many church leaders would become wolves introducing all types of heresies. Judaism (mutilation of the flesh) would be one of them to send Christians to damnation.
Act 20:29-31 For I know this, that after my departure savage wolves will come in among you, not sparing the flock. Also from among yourselves men will rise up, speaking perverse things, to draw away the disciples after themselves. Therefore watch, and remember that for three years I did not cease to warn everyone night and day with tears.
Synagogues of Satan
Rev 2:8-9 "And to the angel of the church in Smyrna write, 'These things says the First and the Last, who was dead, and came to life: I know your works, tribulation, and poverty (but you are rich); and I know the blasphemy of those who say they are Jews and are not, but are a synagogue of Satan."
When God condemned this church (gathering of Christians) to be the synagogue of Satan, then the worships in this church are abominable to Him. Christians must worship God in Spirit and in truth (in the presence of our Lord Jesus), so this church must be like synagogues, which practise some sort of Judaism or Jewish cultures, sacrifices and feasts, which are cursed and are abominations to the Lord.
Since our Lord Jesus had fulfilled all Jewish sacrifices, feasts, Sabbaths, law, etc. He cannot be present whenever the church practises any part of the Law or Judaism - the sign of unbelief of His fulfillments.