How are they non-Jewish? There wasn't just Judaism during that time. It was more like there were multiple Judaisms circulating at that time. We have people from Philo promoting somewhat of a hellenistic form of Judaism, to Paul promoting a more encompassing form of Judaism.
With all respect, fallingblood, I don't think it's quite the same thing. I agree, Philo's Hellenized Judaism is, by Rabbinic standards, nearly as heretical as early Christianity, but Philo, at least was trying to achieve a balance between Hellenism and Judaism that favored Judaism. He did, technically, defend keeping the commandments and remaining monotheistic, and so forth, even if he did do so in ways not acceptable to Rabbinic Judaism at that time (although he gets used by some rabbis way later, post-Renaissance, to some degree).
Now, it seems pretty clear that Jesus, though he was something of an ecletic in his Jewish ideas, drawing from Pharisaic and quasi-monastic ascetic Judaisms, along with his own ideas, was much closer to Pharisaic thought that he was to Hellenic Judaism as it stood in Exilic communities at that time, or to Saducceic Judaism, or even to Qumrani/Essene Judaism. He uses midrash like a Pharisee; his ritual practice is judged even in the synoptic gospels according to its variance with Pharisaic Judaism, not with the other models and traditions; his modes of address and teaching are Pharisaic-- the Rabbinic tradition itself names him as an expelled student of Rabbi Yehoshua ben Prachyah, and they certainly would have been delighted to name him as an Essene or a Saducee or an Apikoros (Hellenistic hereitc, Aramaicized after the term "epikureisti") if they had thought him as such. It is, IMO, quite reasonable to assume that he conceived of Judaism as something relatively close (at least within theological/ritual practical shouting distance) to Pharisaic Judaism, except, of course, more ascetic, perhaps more apocalyptic, and probably actively messianic.
Paul, however, actively becomes apostatic. He doesn't merely introduce syncretisms, but actively tears down structures of Jewish thought and replaces them with other, non-Jewish ideas, wellnigh expressly for the purpose of opening Christianity to non-Jews. What Paul does is antithetical not only to Pharisaic Judaism, but to Saducceic, Essenic, Qumrani, even to Philo's nebulous Hellenicized Judaism.
Jesus may have been heretical by Rabbinic Jewish standards, but at least he was, in his way, doing just what other charismatic reformers (including the prophets) had tried to do, and tried to do after him: that is, to encourage Jews to practice Judaism in a more spiritually energized way, and to try to raise up the downtrodden a little, make the world a better place. That is deeply Jewish, and not at all foreign to any of the Jewish traditions that were vying for the main stream at that time. What Paul did is deeply un-Jewish, and entirely foreign to all of the Jewish traditions that were vying for the main stream at that time.