Recall what Jesus says about the Jews/Pharisees: "All too well you reject the commandments of God, that you might keep your tradition" (Mark 7:9). Paul is saying the same thing, only in more detail and later in time. However, he adds something to it: ". . . they have a zeal for God, but not according to knowledge." This summarizes their zealous attitude and its result. Despite their learning, they were ignorant and established their own righteousness.
With a great deal of zeal, they went about thinking and saying they were serving God, but all they did was produce their own set of standards. Their zeal was probably their greatest barrier to the truth. The apostle Paul is a prime example. Before conversion, he was a zealous Pharisee! He was so zealous that it was notorious to the first-century church; it was afraid of him!
If he is an example of Pharisaism, then we see that his zeal for Judaism drove him to consider Jesus and the church as traitors to his way of life. Simultaneously, it prevented him from objectively examining what was being taught. His mind was blinded by his zeal. It took him getting knocked down on the Damascus Road to convince him otherwise. It is a good example of the miracle that takes place in our minds, though ours is not as dramatic as his. Once converted, Paul could effectively examine the Jews' problem.