The Crucial Sign
Answer: The Chafetz Chaim compared the "sign" between G-d and the Jewish people, which is how the Torah describes Shabbat, the sacred day of rest, to the sign that a merchant has on his shop. The merchant may have some slow times in his sales but as long as the sign is in front of his shop people know that he is still in business. Once the sign is down it means that his business has collapsed.
In similar fashion a Jew may be derelict in some of his religious responsibilities. But as long as he observes Shabbat he is displaying a sign that he is “still in business” as a Jew. Abandoning the observance of Shabbat is tantamount to removing the sign and “resigning” as a Jew.