Brothers, my heart's desire and prayer to God for them is that they may be saved. For I bear them witness that they have a zeal for God, but not according to knowledge. For, being ignorant of the righteousness of God, and seeking to establish their own, they did not submit to God's righteousness. For Christ is the end of the law for righteousness to everyone who believes.
I may be wrong in this, but it seems that there has been a culture shift in our day since the time of Paul. The Jewish people had been steeped in a view of the Old Testament law that was based upon obedience. Even though there has always been a remnant of believers who trusted in the coming Messiah by faith, the majority believed that obedience to God's rules is what earned them favor in God's eyes. They were self-righteous and proud because of their accomplishments over other people. They were ignorant of the righteousness of God, and sought to establish their own. They believed that they needed to do something.
In our "Christian" culture, we have been accustomed to hearing that salvation is through Jesus. We have heard that Jesus paid it all, so I'm good to go. We have not listened to the rest of the song that tells us all to Him I owe. We may still have an inkling inside us that believes that we must do something in order to earn God's favor, and the modern easy believism of decisional regeneration gives us all we need. It tells us that if we just say this prayer, Jesus will forgive us and grant us eternal life. We have nothing left to do now because Jesus paid it all after I chose to ask him into my heart.
The Jewish folk Paul is referring to had a zeal for God but not according to knowledge. The professed church in America has a zeal for God as well, but likewise, not according to knowledge. They may have a zeal for missions, but not according to knowledge. They may have a zeal for the Holy Spirit, but not according to knowledge. They may have all sorts of zeal, but not according to true knowledge.
Let's not be like the Jewish people of old who had a zeal for the law of God, but hated his son Jesus. Let's not be like the modern church who has a zeal for the spirit of God, but not for the God or Son of the Spirit. Our zeal needs to be according to the full and complete knowledge of God; from Genesis to Revelation.