No he wasn't. As far as Judaism is concerned, one can be an atheist and be a Jew, but one cannot believe in Christ and be a Jew.
The author was an apostate Jews who no longer lived like a Jew, and who believed that "man can become God."
The way the VERY early fathers (Peter, Paul, James, etc) looked upon it, they viewed two groups, the circumcised and the uncircumcised; the Jew and the Gentile. Gentiles are grafted into true Judaism which is based upon faith, not circumcision. It doesn't matter what "Judaism" thought.