The Sources frequently tell us that if "all Israel" did this or that, wonderful things would happen. For example, if all Israel observed just one Shabbat properly, the Messiah would come. [Ta’anit Y 1:10; Ex. R. 25:12]
The problem is this: We don't always know who is Jewish and who isn't. It is likely that the vast majority of halachic Jews today have no idea they are Jewish. A single Jewish woman who was forced to convert out a thousand years ago could have thousands of matrilineal descendents today, all halachically Jewish, all ignorant of the fact.
So, given this reality, how do we give meaning to the phrase "all Israel"?