The Rambam makes it clear in NUMEROUS places that ALL Jews [even evil doers - with a few notable exceptions] and ALL righteous Gentiles receive a portion in the world to come. [See, for example, Hilchos Teshuva 3-5 until the end of the Perek].
On the other hand, the Rambam is quite clear that Olam Haba only goes to Jews who are TZADIKIM!!! [See Issurei Biah 14/1 and Teshuva 8/1-2 where he says that one must KNOW Hashem to merit Olam Haba and in the last perek he identifies knowledge with love. Implying that only those who are overcome with intense love of Hashem merit the world to come].
Resolution - here.
