That is, zeal for God. Compare the phrase “the zeal of thine house,” meaning “the zeal for thy house” (Ps. 69:9; John 2:17). The Jews prided themselves upon their zeal for God and for His law (Acts 21:20; 22:3; cf. Gal. 1:14), and Paul has well described their ardor in matters pertaining to religion during that period.

 The sad history of the Jews is that, in spite of their great religious zeal, they did not attain to righteousness (Rom. 9:30-32). Their religion was legal and formal to the last degree. Their outward show of scrupulous obedience was a cloak for inward corruption (ch. 2:17-29). Yet Paul seems to be speaking of their zeal for God as something which in itself is commendable, and, as in ch. 1:8, he points to a good quality first before going on to discuss their failures. He appears to find in this misplaced zeal some ground for encouragement, some hope that if such zeal could be directed toward the true way of righteousness, they might yet be saved.

 Or, “I bear them witness.” Paul was well able to do this from his own sad experience. He himself had once been “more exceedingly zealous of the traditions of my fathers” (Gal. 1:14), and thus was well acquainted with their misguided zeal (see Acts 22:3; Phil. 3:6).

 Gr. epignōsis. This word denotes a full and thorough knowledge (cf. chs. 1:28; 3:20). The Jews were not without knowledge as such (gnōsis), but they lacked the true knowledge that could have led them to serve God in the right way. They had been specially favored with knowledge about God (ch. 3:1, 2), but their zeal for Him had not been intelligent. Though they knew the writings of the law and the prophets, they had no true insight into the meaning of God’s words and works. Their unenlightened ardor became mere fanaticism, and they showed more zeal for the form and the letter than for God.