They have a zeal of God - They believe their law to have come immediately from God himself, and are jealous of its glory and excellence; they conscientiously observe its rites and ceremonies, but they do not consider the object and end of those rites; they sin more through ignorance than malice; and this pleads in their excuse. By this fine apology for them, the apostle prepares them for the harsher truths which he was about to deliver.

I bear them. By this fine apology for the Jews, the Apostle prepares them for the harsher truths which he was about to deliver.

2 Corinthians 8:3 For to their power, I bear record, yes, and beyond their power they were willing of themselves;

Galatians 4:15 Where is then the blessedness you spoke of? for I bear you record, that, if it had been possible...

Colossians 4:13 For I bear him record, that he has a great zeal for you, and them that are in Laodicea, and them in Hierapolis.

that they.

2 Kings 10:16 And he said, Come with me, and see my zeal for the LORD. So they made him ride in his chariot.

John 16:2 They shall put you out of the synagogues: yes, the time comes, that whoever kills you will think that he does God service.

Acts 21:20,28 And when they heard it, they glorified the Lord, and said to him, You see, brother, how many thousands of Jews there are which believe...

Acts 22:3,22 I am truly a man which am a Jew, born in Tarsus, a city in Cilicia, yet brought up in this city at the feet of Gamaliel...

Acts 26:9,10 I truly thought with myself, that I ought to do many things contrary to the name of Jesus of Nazareth...

Galatians 1:14 And profited in the Jews' religion above many my equals in my own nation, being more exceedingly zealous of the traditions of my fathers...

Galatians 4:17,18 They zealously affect you, but not well; yes, they would exclude you, that you might affect them...

Philippians 3:6 Concerning zeal, persecuting the church; touching the righteousness which is in the law, blameless.

but not.

Romans 10:3 For they being ignorant of God's righteousness, and going about to establish their own righteousness...

Romans 9:31,32 But Israel, which followed after the law of righteousness, has not attained to the law of righteousness...

Psalm 14:4 Have all the workers of iniquity no knowledge? who eat up my people as they eat bread, and call not on the LORD.

Proverbs 19:2 Also, that the soul be without knowledge, it is not good; and he that hastens with his feet sins.

Isaiah 27:1 In that day the LORD with his sore and great and strong sword shall punish leviathan the piercing serpent...

2 Corinthians 4:4,6 In whom the god of this world has blinded the minds of them which believe not, lest the light of the glorious gospel of Christ...

Philippians 1:9 And this I pray, that your love may abound yet more and more in knowledge and in all judgment;

Christ the End of the Law
The reason why many do not come to Christ is not because they are not earnest, after a fashion, and thoughtful and desirous to be saved, but because they cannot brook God's way of salvation. "They have a zeal for God, but not according to knowledge," We do get them by our exhortation so far on the way that they become desirous to obtain eternal life, but "they have not submitted themselves to the righteousness of God." Mark, "submitted themselves," for it needs submission. Proud man wants to save …
Charles Haddon Spurgeon—Spurgeon's Sermons Volume 22: 1876

Which Sentence Dishonoreth the Holy Martyrs, Nay Rather Taketh Away Holy Martyrdoms Altogether. ...
3. Which sentence dishonoreth the holy Martyrs, nay rather taketh away holy martyrdoms altogether. For they would do more justly and wisely, according to these men, not to confess to their persecutors that they were Christians, and by confessing make them murderers: but rather by telling a lie, and denying what they were, should both themselves keep safe the convenience of the flesh and purpose of the heart, and not allow those to accomplish the wickedness which they had conceived in their mind. …
St. Augustine—Against Lying

Or Haply is it So, that He who Plots in this Way to Find...
13. Or haply is it so, that he who plots in this way to find out Priscillianists, denies not Christ, forasmuch as with his mouth he utters what with his heart he believes not? As if truly (which I also said a little above) when it was said, "With the heart man believeth unto righteousness," it was added to no purpose, "with the mouth confession is made unto salvation?" [2398] Is it not so that almost all who have denied Christ before the persecutors, held in their heart what they believed of Him? …
St. Augustine—Against Lying