Halachically, when we need to define "who is a heretic" for laws like minyan, conversion, handling wine, etc... we follow Rabbi Yosef Albo (Iqarim, 1:2) Rav Shimon ben Tzemach Duran (Ohiev Mishpat 15a) the Radvaz (Responsum 1248), and apparently the Raavad (note on Laws of Teshuvah 3:1), among others, who say that it is not belief in heresy that makes one categorizable as a heretic, but rebelling against G-d to the point of embracing heresy that does.
And so, the dominant belief with regard to the afterlife is correspondingly that someone seeking Truth but through no fault of his own reaches the wrong conclusion is not a heretic and not deprived of a place in the World to Come. We tend to see human perfection in terms of morality, ethics, and culpability in general. Not "does he believe heresy" but "is it his fault?"
This is true of a Jew who is misled by reading the scriptures overly literally (the Radvaz and Raavad's case), one who was raised by people with a different belief system, or, I presume, the non-Jews in question.
According to the Rambam, personal redemption is a product of knowledge. He thus opens the Guide to the Perplexed with a discussion of how pre-sin Adam had to distinguish between truth and falsehood, and the need to distinguish between good and evil is a consequence of that first sin. In 3:18 we learn that humanness is proprotional to such knowledge, and therefore Divine Providence is as well. 3:27 describes prophecy as a step beyond philosophy. (See also 3:31, his explanation for mitzvos, and elswhere for more of the same.) And in the closing paragraphs, he ranks perfection of knowledge above moral perfection.
And so it's consistent for him that he writes that proper knowledge is what causes reward in the afterlife (Laws of Teshuvah ch. 8). It's knowing about the Eternal One that grants one eternity (Commentary on Mishnah, introduction to Sanhedrin ch. 10). It's a causal thing, and therefore guilt and cupability don't really enter into it. (Beyond saying that someone who doesn't act G-dly wouldn't possibly know Him.)
But this is a minority view, and not what any stream of contemporary Judaism that I know of still preaches.
And even the Rambam, who would say that the wrong belief itself causes a lack of reward in the World to Come, would not say necessarily that it must cause its absence altogether. After all, he credits Islam and to a far lesser extent Christianity as steps along the way to the world reaching the truth. (Laws of Kings 11:10-11) With more truth, one gets more reward. Proportion, rather than all-or-nothing.
(This depends on a long-standing debate about how Maimonides defines "gehennom". There is indication (Commentary on Mishnah, ibid) that he may believe that the ultimate punishment isn't "hell", but a cessation of existence. If that is indeed his position, then a proportional resolution isn't likely.)
