The passuk says ובכל אשר אמרתי אליכם תשמרו ושם אלהים אחרים לא תזכירו לא ישמע על פיך. Rashi explains that the first part of the passuk gives each and every mitzvas aseh a warning of a lo saseh. The second part of the passuk teaches that one shouldn’t say to watch or wait near a certain avodah zara, or stay with me on a certain day of an avodah zara. Rashi brings another explanation; one who is careful to not do avodah zara is as though he kept the whole Torah. The passuk teaches us that keeping away from avodah zara is equal to the whole Torah!
The Ohr HaChaim HaKadosh asks: the passuk says תשמרו (with a tzeirei under the mem) and not תשמורו. Shouldn’t it say “you shall watch” and not “you shall be watched”? He explains with a Gemara in Makos that says there are 248 mitzvos aseh corresponding to the 248 limbs of a person, and 365 mitzvos lo saseh which correspond to the 365 sinews. The passuk is hinting to us that one should fulfill all the mitzvos and not just certain mitzvos, in order to have complete protection. The passuk says, “and with all that Hashem said… with all the mitzvos you shall be watched.” However if a person is missing one mitzvah he will be missing the protection for the one limb or sinew which it corresponds to.
The second part of the passuk speaks about avodah zara. The Ohr HaChaim HaKadosh explains that just as keeping all the mitzvos brings a protection for the whole person, so to with keeping away from avodah zara. If one does not worship avodah zara it is as if one is admitting to the whole Torah and gets the protection of all the mitzvos. Each and every limb and sinew has protection even without doing all the mitzvos! As long as one isn’t mivatel a mitzvah intentionally.
On Shabbos we say “גם עבדך נזהר בהם בשמרם עקב רב” The Nefesh HaChaim brings that נזהר means bright from “zohar” shine, and עקב means the heel. By one doing mitzvos and learning Torah he will bring light into his whole body, even until the heel which is the most geshem part of the body.
May Hashem help us watch the mitzvos aseh and lo saseh to bring protection and light to our whole body!