Actually, the story of Phineas, as it concerns him, is all about being zealous for keeping the law.
I heard a theologian Dr. Michael Eaten say that by the time Paul went to Jerusalem in Acts 21...most of the pure gospel had been lost as thousands of Jewish believers had been zealous for the law...it doesn't take long for Judaizers to come in and pervert the gospel of the grace of Christ alone.....Paul said they would come right after he had been to places with their "works-based " gospel..
Acts 21:20 (NASB)
[SUP]20 [/SUP] And when they heard it they began glorifying God; and they said to him, "You see, brother, how many thousands there are among the Jews of those who have believed, and they are all zealous for the Law;