That was Paul's nice try to think that he would remain a Jew after he founded an anti-Jewish religion. According to Jewish tradition, when a Jew deserts Judaism to become a member of another faith, especially so anti-Jewish as Christianity has been to the Jews throughout History by means of pogroms, blood libels, Crusades, Inquisition and the Holocaust he ceased being a Jew. Paul was no longer a Jew from that day in Antioch when Christians started being called Christians for the first time as a result of Paul's teaching for a whole year in the Nazarene Synagogue of the Jews. Read Acts 11:26.
There is quite a bit of debate at this board about whether Christianity is a wholly different religion from Judaism, or not. Also about when/who created the split.
For my part, it certainly appears that Jesus did not create a new religion, but was rather seeking a reformation within Judaism. His core teachings include
(1) an abrogation of the vast and unwieldy body of case law (much of Oral Law) that had been added to Torah,
(2) an elevation of the Rabbi and the obsolence of the corrupt priesthood,
(3) a vastly simplified way of interpreting Torah (e.g. according to intended results),
(4) a dereliction of patriarchal genealogies for the purpose of establishing Jewish blood, (being replaced by a system of judging on merit as regards Torah observance), and
(5) the abrogation of the entire sacrificial system, in favor of a once-for-all sacrifice.
Note that Judaism actually had reforms in the decades after his death in most of these areas. (2) was reformed exactly as noted above. Item (4) was reformed later, but with a system different than proposed. Items (1) and (3) were addressed through the addition of diacritical marks to the Torah, eliminating the vast majority of diverse readings.
As for Paul, his teachings
to Gentiles do not appear to me to be a new invention, either, but rather a restatement (albeit an unnecessarily verbose and complex restatement) of the traditional teachings regarding goyim upholding righteousness. There is some blurriness between Jesus' (4) above and Paul's teachings, and I am not sure whether to attribute that to Paul by design, or to put it down to a corruption of his teachings by his followers after his death.