Why did the Holy Spirit inspire Paul to say he was unknown to the churches in Judea?
The Spirit inspired that statement to show Paul's ministry was completely separate from the Jewish congregations in Palestine. Paul was sent to Gentiles, not to Judea, forming a fork in the road from Peter and the Twelve, who were not sent to Gentiles.
Les explains that after Paul's two weeks with Peter, he went from Jerusalem directly to his home area of Cilicia, which was total Gentile territory. The Holy Spirit had Paul make that statement because Paul was going to be separated from all that. He had no ministry to the Jews in Judea, just as the twelve had no ministry to the Gentiles when Jesus sent them out. It was a complete fork in the road: Paul was sent in the opposite direction from Peter and the apostles, into a total new Gentile area, Cilicia and Tarsus. Though Jews were scattered throughout the Roman Empire, Paul's calling was distinct from the Jewish congregations in Palestine.
And was unknown by face unto the churches of Judaea which were in Christ: