Although the work is not gnostic in a theological sense, the author, who considers himself to be a teacher to the unidentified audience to which he writes (see e.g. 9.9), intends to impart to his readers the perfect gnosis (knowledge), that they may perceive that the Christians are the only true covenant people, and that the Jewish people are no longer in covenant with God (3.7). His polemics are, above all, directed against Judaizing Christians (see Ebionites, Nazarenes, Judaizing teachers).

Based on this passage, what is the main message of the epistle of barnabas? Let's give stream of consciousness first:
To answer this question, we should know that: Although the work is not gnostic in a theological sense, the author, who considers himself to be a teacher to the unidentified audience to which he writes (see e.g. 9.9), intends to impart to his readers the perfect gnosis (knowledge), that they may perceive that the Christians are the only true covenant people, and that the Jewish people are no longer in covenant with God (3.7).
So, the final answer is the perfect gnosis ( knowledge ) , that they may perceive that the Christians are the only true covenant people , and that the Jewish people are no longer in covenant with God ( 3.7 ).