http://revach.net/article.php?id=1884

Section: Parshas HaShavua   Category: Quick Vort
Parshas Ki Sisa: Chofetz Chaim - Shabbos, If You Take Down The Sign....

Why is Shemiras Shabbos considered the litmus test if a person is religious or not? Hashem says regarding Shabbos (31:13) "Ois He; It is a sign." The Chofetz Chaim explains with a Mashal of a sign on a person's store. "Every business has a reputation and the name on sign tells the whole world who 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 rest assured 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 with no intention of returning.

Similarly with Shemiras HaMitzvos. Keeping Shabbos is a sign that the Jewish spirit resides in the person. Even if a Jew leaves the path, as long as he keeps Shabbos, we know the Ruach HaTorah is still in him and he will one day return. However once a person is Michalel Shabbos, he has taken down his sign and we know that the Ruach HaTorah has left them."