I once met a young man who believed the Written Torah was from Heaven but who rejected the rest of the Bible (as well as the Oral Torah) and even rejected the doctrine of an afterlife. While he certainly was not an Orthodox Jew, I fail to see why his belief system failed to qualify as a "branch of Judaism" just as much as "reform," "reconstructionism," and "secular humanistic Judaism."
This is a due to the arrogance of the more established Jewish communities.
Halakhically, the Jews are not so much a "religion" as a nation that has a covenant with G-d. A Jew who apostasizes to another religion is an apostate Jew and is subject to the Halakhic penalties prescribed for Jews who apostasize.
Meanwhile all non-Jews, regardless of religion or lack of it, are Halakhically Noachides, and thus any practicing Noachide who apostasizes to another religion is an apostate Noachide and subject to the Noachide penalties for apostasy.
What we both agree on is that a Jew who apostasizes to chr*stianity is not a "good Jew" or a "completed Jew" or a "true Jew" or any such thing. However, in the eyes of HaShem he remains a Jew and subject to penalties prescribed for Jews who apostasize.