Scot McKnight asks the question, and points to Jimmy Dunn’s new book for a possible answer:
“It was as an Israelite that Paul was an apostle to the Gentiles, and as an apostle to the Gentiles Paul was fully an Israelite. Paul was no apostate; he was an apostle of Christ and for Israel. Dunn also develops the eschatological perspective on Paul, namely that the curtain of history was coming down and he was playing a role in that drama. He appeals to 1 Corinthians 4:9, Romans 11:13-15…”
This is true enough as stated but only gets at part of the issue, and perhaps not the more significant one. Whenever scholars ask, “Was Paul an apostate?”, or “Was Paul a convert?”, or “Was Paul sectarian?”, seldom enough emphasis falls on the reception of Paul’s gospel, which is what really matters. Apostates naturally think they’re faithful, and often show themselves brilliantly capable of using tradition to justify whatever they need. The proof of the pudding is in the eating, or in the reception, and Paul went against enough opposition and hostility to convince other scholars, contra Dunn, that it’s perfectly reasonable to speak of Paul’s conversion more than calling, sectarianism instead of renewal, and apostasy trumping apostolateship.
In my view, candid texts like Philip 3:4b-11 and II Cor 3:4-11 carry far more weight in answering the question than texts like I Cor 4:9 and Rom 11:13-15 (in which Paul simply calls himself an apostle), or Jer 1:5 and Isa 49:1-6 (where Paul claims continuity with prophetic tradition, naturally begging the question as to what it means to be an appropriate “light to the nations”). Of course, I’ll have to read Dunn’s book; he may allow for more nuances than I’m granting him here. According to McKnight, Dunn reflects on the way the deutero-Pauline letters had to mollify or sanitize Paul’s image, though seems to dodge the implications by throwing the spotlight on modern believers, and claiming that the problem resides with them or anyone else who can’t appreciate Paul’s continuity by “listening to the spirit”.