John’s Opponents, again

In his commentary on the Johannine epistles, I. Howard Marshall notes A. Wurm’s thesis that John’s opponents were Jews. John’s opponents claim to know the Father, but deny Jesus is the Christ, a position that is certainly compatible with Judaism. Marshall dismisses Wurm’s thesis because John treats his opponents as people who had been members of the Christian community (citing 2:19), and therefore cannot be non-Christian Jews.

Which makes sense, perhaps (though Marshall’s treatment of 2:19 is open to question). But that doesn’t really finish the argument, since, as Brown acknowledges, one might then argue that the opponents were Jews who had been Christians and lapsed.