Acts 15:4

And when they were come to Jerusalem, they were received of the church and the apostles and elders, and they rehearsed all things that God had done with them. But there rose up certain of the sect of the Pharisees who believed, saying, It is needful to circumcise them, and to charge them to keep the law of Moses.

The Pharisees who believed ... Here is identified the seat of the mischief. In Acts 6:7, Luke related how a great company of the priests believed, many of whom no doubt were Pharisees. Their love of the forms and ceremonies of Judaism had been brought with them into the church; and it may be assumed that for some considerable time they had been working to graft their own system into Christianity. Not only had they corrupted practically the whole of the church in Judea, but the recently established churches in Galatia had been visited and corrupted sufficiently to call forth Paul's vehement letter to the Galatians. The representatives they sent down to Antioch probably expected a quick victory there also; but instead of a victory they suddenly confronted the dauntless Paul who challenged them, defeated them, and proceeded to Jerusalem where he reversed the victory they had already won there. Aside from Christ himself, Christianity owes more to Paul than to any other.