Paul had been a Pharisee, and knew all about the law. However, once he was saved, and indwelt by the Holy Spirit (at the same time, Eph. 1:1313In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, (Ephesians 1:13)), he came to know the total rejection of God by the Jews. (1) They rejected Jehovah; (2) they rejected the Christ of God, Who came as Messiah; (3) they rejected God, the Holy Spirit (Acts 7).
V.14 The Jews had a zeal (energy to serve) God, but not the right way. They were going to establish their own righteousness. And just think of how many people there are today who are trying to do their best.
V.5-13 The gospel is easily received and is open to all.
V.6-7 No one needs to think that they have to go a long distance (up to heaven) or down into the abyss to receive Christ. Salvation is close, ready for anyone.
V.8-13 The lips to say, the heart to believe are all that is necessary on our part.
V.14-21 God has given the Jews every opportunity, and has done everything possible. There are four links in a chain between God and them: the apostles, preaching, hearing and faith (believing). But they broke the chain, they would not believe.