72:1 Shabbos is a great sign and a covenant that G-d gave the Jewish people testifying that He made the Heavens and the Earth and all they contain in six days, "resting" on the seventh day. This is a basic tenet of Judaism. The Talmud Yerushalmi (Nedarim 3) equates Shabbos with all the other mitzvos combined. One who properly observes Shabbos is considered as if he observed the entire Torah, while one who desecrates Shabbos is considered as if he violated the entire Torah. Ezra said likewise: "You (G-d) descended on Mount Sinai and gave Your people the Torah and mitzvos, and You made known to them the holiness of Your Shabbos…" (Nehemiah 9:13-14).
72:2 One who (regularly – Mishnah Brurah 385:4) desecrates Shabbos in public (even in a rabbinic prohibition – Orach Chaim 385:3) has the same status as an idolater under Jewish law: if he touches wine, it is forbidden; the bread that he bakes is subject to the same restrictions as bread baked by a non-Jew and the food he cooks is subject to the same restrictions as food cooked by a non-Jew. The definition of "publicly" is in the presence of ten Jews. The desecration does not literally have to be performed in their presence, they just have to be aware of it. We see this from the Talmud in Sanhedrin (74b). There it asks whether Esther sinned publicly (by being married to Achashverosh even though she was still legally married to Mordechai - people knew that Esther and the king were intimate in the marital sense, even though nobody saw it). The Pri Megadim also writes that "publicly" means that ten Jews know about a matter. (If one would be too ashamed to violate Shabbos in front of a great person, he is not considered a public desecrator – MB 385:6.)