Exodus Rabbah 14:3
Why did the Holy One bring darkness upon the Egyptians? Because there were transgressors in Israel who had patrons among the Egyptians, lived in their midst in affluence and honor, and were therefore unwilling to leave Egypt. The Holy One said: If in the sight of all I bring a plague upon them also and they die, the Egyptians will say, "Just as plagues have befallen us, so has a plague befallen them." Therefore He brought darkness upon the Egyptians for three days in order that they [the Israelites] could bury their dead.
Apparently the wonders and miracles of the plagues were not enough to inspire all of the Israelites to want to leave Egypt. Moreover, according to this midrash, not all of the Israelites were slaves. There were some who had reached positions of assimilated success, wealth, and honor who enjoyed their positions in society. In spite of their class differences, God wanted the Israelites to act in unity. That there were some in society who did not want to leave Egypt, or who were not suffering as their enslaved brethren were, was irrelevant. God was so serious about this that a mortal plague struck those who would have chosen to stay behind. Rather than create that misimpression that the death of those few Israelites was by chance or nature, God sent darkness to cover the affair.
Reading this midrash as American Jews today, we must ask: what, in our day and age, is considered the most offensive act a Jew can commit against the Jewish people? Not so long ago, and still today in some corners, the answer would have been (or continues to be) marrying someone who is not Jewish and does not plan on converting. I do not think this is the case for most of our communities any longer. Certainly the custom of sitting shiv'ah for those who "marry out"—which might have been done under cover of darkness if possible—is no longer the norm for our communities, which are preoccupied instead with creating spaces that welcome mixed-marriage families. With intermarriage out, what would that most terrible of actions be?
It seems that if we can identify the boundary of the norm, so that we can say what is outside the norm and what is not, we might then learn about what it means to be a bona fide member of the Jewish community.
To my mind, the defining boundary is the one around ethics. For better or worse, there have been some highly public cases in the last several years of Jews who held positions of "wealth and honor" (as the midrash would have it) who were found to be defrauding the public. I will not lend them more publicity here. In thinking about the conversations that ensued within the Jewish community each time such a story made the news, however, it seems that those acts have been the ones where the community has whispered, Oy, did he (she/they) have to be a Jew? Honesty in business and all interpersonal matters is the bedrock of Jewish teaching. When someone acts against that foundational value, it comes to disgrace the entire community—and we find ourselves wishing we had the cover of darkness to deal privately with the problem. That which gave Judaism its spiritual force from the outset was the prophetic message of ethical living. We must never lose sight of that message of honest living which is at once the most humble and most lofty of Judaism's teachings, and which is what, at our core, should bind us as a people.