In my book, I argue that the ζῆλ* (“zeal”) word-group in Paul and in Acts should be understood as a “divine jealous passion” for the purity of the people of Israel and their Law, with Phineas (Numbers 25:1-13) as the prime exemplar. “Since God is ‘jealously passionate’ for Israel, he expects Israel to be exclusively committed to him” (pp. 90-91).
Such jealous passion for the purity of the people and the Law is … depicted by others as a commendable commitment to Israel’s divine vocation in scriptural terms. It is not merely an expression of nationalistic “exclusivism”—that is, a desire to keep God’s blessing away from the Gentiles. Rather, it is an expression of Israel’s sense of divine vocation. As a “kingdom of priests,” Israel’s purity and separateness from the nations is a key part of their task to display God’s glory to the nations.
Paul himself displayed this kind of ζῆλος before his conversion as he persecuted the “assembly” (ἐκκλησία), presumably because he feared that the proximity of large numbers of Gentiles to Jews within this concrete “assembly” would compromise Israel’s purity (Gal 1:14, Phil 3:6). (pp. 91-92)
I argue that simply translating the word-group with “jealous” (in the sense of being envious) often leads us astray, especially in Acts:
Cf. the references to Jewish ζῆλος in Acts (5:17, 13:45, 17:5). Given the Jewish background to the concept, it is highly unlikely that Acts is depicting the Jews as somehow “jealous” of the Christian missionaries—as if the Jews wished that they had brought all those impure people (sick, demoniacs, and Gentiles) into their midst! Rather, like the Maccabees, they are “passionate for the Law” (cf. Acts 21:20–21), and thus fearful that Israel’s purity and devotion to God is being compromised by the unprecedented influx of “impure” people resulting from the apostolic preaching. They are not envious of the influx of Gentiles: they want to stop it! This is why Acts is at pains to point out that God has made the Gentiles “clean” through faith and the Spirit, thus rendering the Jewish fears illegitimate (Acts 10:28, 15:9). (p. 92 n. 223)
Why did Paul reject his former “zeal”? Because of the gospel.
The gospel of Christ, however, radically changes Paul’s evaluation of this attitude. The gospel shows that Jews are in fact on the same level as Gentiles in this regard—Jews do not constitute a holy people in need of protection against contamination, but rather are “sinners” in need of justification (Gal 2:15–17; cf. Rom 10:2). Thus, in light of the gospel of Christ, Paul rejects ζῆλος as an illegitimate expression of Jewish vocation. (p. 92)
The details of the argument and further references may be found in chapter 3 of the book (pp. 90-93). The chapter is available from the publisher in electronic format:
Windsor, Lionel J. Paul and the Vocation of Israel: How Paul’s Jewish Identity Informs his Apostolic Ministry, with Special Reference to Romans. BZNW 205. Berlin / Boston: Walter de Gruyter, 2014.