The sign

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Why is Shemiras Shabbos considered the litmus test if a person is religious or not? Hashem says about Shabbos (31:13) "Ois He"; it is a sign. The Chofetz Chaim gives Mashal to a sign on a person's store. Every business has a reputation and the name on sign tells the whole world the owner of the store is. Even if the owner travels away for a while, as long as the sign is still up we can count on the fact that the owner will eventually return. If the owner takes the sign down and leaves town then it is clear that the owner has left and has no intention of returning.

Similarly with Shemiras HaMitzvos. Keeping Shabbos is a sign that the Jewish spirit resides in the person. Even if they leave the path as long as they keeps Shabbos we know the Ruach HaTorah is still in them and they will one day return. However once a person is Michalel Shabbos they have removed the sign and we know the Ruach HaTorah has left them.
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