On the one side of this dispute we have Paul, who is saying: The gospel of faith in Christ is for people of all cultures. On the other we have his opponents, claiming: Not all Jewish people are Christians, but all Christians must become Jewish.
If the Jerusalem apostles had sided with, or even merely tolerated, those who were teaching against Paul, this would have split the church in two. Neither side would have accepted the other fully, and would have questioned if the others were saved! Paul’s Gentile churches would doubt that the Jewish churches really had faith in Christ, and the Jewish churches would also doubt the salvation of the Gentiles.
John Stott put it this way:
“It was one thing for the Jerusalem leaders to give their approval to the conversion of the Gentiles, but could they approve of … commitment to the Messiah without inclusion in Judaism? Was their vision big enough to see the gospel of Christ not as a reform movement within Judaism but as good news for the whole world, and the church of Christ … as the international family of God?”
The other apostles had stayed in Jerusalem, and they had not worked out the implications of the gospel for Gentiles who were converting from paganism. They simply had not confronted most of these issues practically. It would have been extremely easy for them to miss the implications of the gospel when it came to living as a Gentile Christian. It would have felt natural for them to say: Of course all Christians should eat kosher ! or something similar. But the ramifications of such a “small” mistake would have been enormous. There would have been two opposing parties within Christianity that were hostile to each other on the fundamental point of whether we need to add external behaviors to internal belief in Christ in order to be saved.
That’s why Paul said that “the freedom we have in Christ” (v 4) was under threat, and therefore that the very “truth of the gospel” was at stake (v 5). This meeting could have ended up splitting the church; and at such an early stage in its life, two virtually different religions would have emerged. No wonder Paul felt a certain fear. The stakes could not have been higher.