Greetings. How is it possible that the gemara in Rosh Hashana 17b discusses the existence of gehenom when we know from Pirke Avos that Kol Yisroel Yesh Lo Chelek le’Olam Habah? Thank you.
Gehinnom and Olam Haba are not a contradiction, as if a person can only get one or the other. What the mishna means that Kol Yisroel yesh lahem chelek l’olam haba, is that every Jew is destined (unless he ruins it for himself) to enter a certain place in the world to come for eternity, that has a special closeness to H-shem and a very high level of pleasure. Let’s compare it to a ballroom to a special banquet with the king. However if the person’s soul comes to the world of truth and it is soiled from all sorts of things that it did, it will need to get “cleaned” and cleansed, before it is befitting to enter the special ballroom. That cleansing process is done in a place called gehinnom (and yes, getting dry cleaned in the next world is harder than getting cleanedin this world). Therfore the two ideas are not a contradiciton at all.