I was thinking about the gemara quoted in this post, where Eliyahu Hanavi says that no one in the (presumably Jewish) market that day was a "ben olam habah" except for a warden and a jester who did their work for the sake of Hashem. Although it was not the immediate point of the gemara, there is a glaring kasha on what Eliyahu Hanavi said: How can it be than almost no one in the whole shuk was a "ben olam habah"? Doesn't the mishna in Avos say that "kol Yisroel yesh lahem chelek l'olam habah," that every single Jew has a share in the world to come?
(And I don't think you can answer that the Gemara in Sanhedrin lists a number of exceptions to the general rule because: (a) it's shver to be mokim the gemara that everyone in the shuk were the worst of the worst, baalei lashon hara, apikursim, and mevazei talmidei chachamim and (b) there's a shita in the gemara there that the dorshei reshumos learn that ona deeper level, even the Yidden she'ein lahem chelek l'olam habah really do have a chelek.)
I think the answer lies in the phrase "ben olam habah." It could have referred to "people with a chelek in olam habah." Instead, Eliyahu referred to "bnei" olam habah," implying people who are "in the category of olam habah." Meaning: a ben olam habah is someone who lives in olam habah even in this world. What does this mean? In olam habah, "kol ha'aretz deiah es Hashem," the whole world and all aspects of physicality ("artzius") will know Hashem. Someone who lives in olam habah now knows that connecting to Hashem and doing the ratzon Hashem is the purpose of life in the world, and they don't get confused by the upside down priorities of the physical world ("olam hafuch ra'isi"), although he is indeed living in this world.
That's why a talmid chochom is called "Shabbos." Shabbos is "me'ein olam habah," an aspect of the world to come, and the Tzadik is called Shabbos because he lives with the knowlwdge of Hashem that will exist in the world to come, even in this world. Such people are indeed in the category of a "ben olam habah."
That's why so few people in the marketplace were bnei olam habah. Most of us get lost in the details of olam hazeh. We may have a share in olam hazeh and may, IY"H, be fulfilling basic halacha, but we are not mentally living with olam habah while we go about our business. We're living in the world of bechira and not yediah.
IY"H we should be zocheh to be bnei olam habah, living with olam habah even as we live, for the moment, in olam hazeh.
(Full disclosure: It could be I heard or learned this idea somewhere before, but I don't remember.)
Picture courtesy of math.toronto.edu.