Paul said about himself, "Concerning zeal, persecuting the church" (Phil. 3:6). This reveals the pride of personal devotion to his religious choices. In a sense, it was Paul's pride of reputation. He was more devoted than any of his contemporaries.
He was not only a Pharisee, but he was also a very zealous one. He was a conscientious and relentless persecutor of all who were considered heretics outside of his pharisaic Judaism.
In Paul's unsaved state in Judaism, he actually thought he was doing the will of God by persecuting the believers in Jesus Christ. He measured his religion by his hatred for Christians.
It is regrettable that even today some believers measure their Christian zeal by what they are against.
Some have so much bitterness against modernists--those with liberal theology; others contend zealously over the issue of the Holy Spirit or over a particular translation of the Bible.
Some have bitterness toward sinners, not distinguishing the sin from the sinner. But remember, a reputation of zeal against anything is not a proof of salvation in itself.
I believe that when we are rightly related to Jesus Christ, we will have much zeal against those things that dishonor Him, but it is possible for people to be zealous against some things without having a right relationship with Christ.
"Who gave himself for us, that he might redeem us from all iniquity, and purify unto himself a peculiar people, zealous of good works" (Titus 2:14).