Islam and Christianity (or at least the most common forms of each) seem to have in common this idea that believing the correct thing about the Nature of God, his revealed Holy Books, and perhaps a few other specific beliefs, is a necessary but not sufficient condition to get into heaven or avoid eternal torment or, in some sense, have a better or eternal afterlife. In short, believing the correct thing will affect your rewards in the afterlife.
I think it's worth asking why. If someone was born in a Muslim country, surrounded by Muslims, never even interacted with a Christian, but they lived otherwise virtuously with kindness and charity and generosity and so forth, why does it matter whether they believe certain specific things? Why would the Christian God care that this person never specifically accepted the statement "Jesus Christ is the son of God" (or whatever other set of statements you may think are necessary for salvation)?
This becomes an especially difficult question, in my estimation, when you consider each religion's reliance on "faith" - that you are good for maintaining faith, and bad for leaving or rejecting your faith. This emphasis on faith in these religions has the effect that changing your mind about something is heavily discouraged, which of course means that Muslims have strong pressures to stay Muslim and Christians have strong pressures to stay Christian.
So if the result of the pressures of Faith is to just continue believing whatever it is you have been taught, then that makes it even extra hard for eg a Muslims to turn to Christianity if it turns out Christianity is true, or conversely for a Christian to turn to Islam if that turns out to be true.
This problem would be entirely sidestepped if God, for example, explicitly revealed the truth to each person individually in an unambiguous way. Then, of course, you would have to choose to deny that.
Some people believe that to be the case I suppose - that there is some form of unambiguous reveal in everybody's lives - but I don't believe that's a tenable position.
It might not be immediately obvious to some of you why this question is important, but consider this possibility: the Mormons are right. Consider the possibility that you end up in the afterlife, and you find that you don't have access to the highest level of Heaven, because you didn't get baptised in a Mormon temple and accept their beliefs. The Mormon answer to that is that, if you earnestly pray to ask if the Book of Mormon is true, you will find that it is. However, I invite all readers to make such a prayer right now. I suspect that, of you who receive an answer, those who are already Mormon will mostly receive a "yes" and those of you who are not will mostly receive a "no" - though please report exceptions! So, you pray, you don't get an answer or you get a "no", you've researched the BoM and you rationally conclude that it's probably not true, and yet you end up in some lesser form of afterlife and find that you were wrong. But you've lived your whole life virtuously, accepted Jesus - you might reasonably find yourself asking, "Why did it matter that I didn't accept these specific claims? Especially when all the evidence and my prayers told me otherwise?"
So that's my question. Why would that matter?