2 For I bear them witness that they have a zeal for God, but not according to knowledge.
2 For I bear them witness
Paul personally experienced zeal for God before he became a Christian, and he also saw the zeal for God in his colleagues in Judaism.
that they have a zeal for God,
There is no question about the religious zeal of the Israelites. Paul probably saw himself as a zealot as well. He murdered Christians to advance Judaism. The Jews had a misdirected zeal. They pointed in the wrong direction with zeal about the wrong scheme of salvation.
Acts 22: 3 “I [Paul] am indeed a Jew, born in Tarsus of Cilicia, but brought up in this city at the feet of Gamaliel, taught according to the strictness of our fathers’ law, and was zealous toward God as you all are today.”
but not according to knowledge [full knowledge].
Israel’s zeal was not according to the knowledge of God’s absolute righteousness. The Jews had a relative view of His righteousness and were ignorant of His perfect righteousness. They had a partial knowledge of God but not a proper view of His nature that depends on a particular kind of salvation. Human righteousness cannot save a soul; we need divine righteousness. Since the standard is beyond human capacity, God must grant His righteousness as a gift to the one who believes.
Good intentions is no excuse for rejection of Christ.
Zeal is neither good nor bad in itself. If zeal does not rest on knowledge of biblical values, it can be a problem. Zeal can carry a bad end if it is exercised wrongly. It can end in disaster from an eternal perspective point of view. There is no intrinsic value in sincerity, for it is possible to be sincerely wrong. Religious people can be sincerely wrong about their beliefs.
Ga 1: 14 And I [Paul] advanced in Judaism beyond many of my contemporaries in my own nation, being more exceedingly zealous for the traditions of my fathers.
Ph 3: 5 circumcised the eighth day, of the stock of Israel, of the tribe of Benjamin, a Hebrew of the Hebrews; concerning the law, a Pharisee; 6 concerning zeal, persecuting the church; concerning the righteousness which is in the law, blameless.
Spiritual blindness is the basis for unbelief. People stumble over Christ because Satan has blinded their eyes to the gospel. Lost religious people do not lack zeal; they lack proper understanding of the gospel. Without knowledge, zeal can lead us astray. This is especially true when it comes to the doctrine of justification. God will declare us as right as He is right only by faith.