This puzzles me a bit.

Acts 22:17-23(NKJV)

“Now it happened, when I returned to Jerusalem and was praying in the temple, that I was in a trance 

and saw Him saying to me, ‘Make haste and get out of Jerusalem quickly, for they will not receive your testimony concerning Me.’ 

So I said, ‘Lord, they know that in every synagogue I imprisoned and beat those who believe on You. 

And when the blood of Your martyr Stephen was shed, I also was standing by consenting to his death, and guarding the clothes of those who were killing him.’ 

Then He said to me, ‘Depart, for I will send you far from here to the Gentiles.’”

And they listened to him until this word, and then they raised their voices and said, “Away with such a fellow from the earth, for he is not fit to live!” 

Then, as they cried out and tore off their clothes and threw dust into the air, 

Why was it such a big deal that Paul went to the Gentiles?  There are many examples throughout Scripture where people from other countries accepted Judaism and were welcomed.  So why would they care if Paul was spreading the Gospel of Jesus, which the Jews who were complaining didn't believe in anyway?