1. Let's define exclusivism as the view that to be saved during the Christian era a mentally competent person must exercise explicit faith in Jesus prior to death. That's controversial, but it's the bedrock of Christian evangelism.
2. According to one version of inclusivism, a person can be saved through a receptive response to general revelation. According to a related version, a person can be saved through implicit faith.
3. Then you have mediating positions that are technically exclusivistic, but are really face-saving versions of inclusivism. For instance, the theory of postmortem salvation, where people can be saved by exercising faith in Jesus after they die. Technically, that might be classified as exclusivism, but it's functionally equivalent to inclusivism. Put another way, it's a radical modification of what exclusivism traditionally meant. In that attenuated sense, even universalism is exclusivistic. At which point the contrast between exclusivism and inclusivism becomes moot.
4. You also have William Lane Craig's conjecture that God has arranged history so that no unreached person would be receptive to the Gospel if given the opportunity. That suffers from several problems:
i) There's no evidence that it's true.
ii) It depends on the dubious theory of middle knowledge.
iii) It conflicts with Craig's belief that:
The hypothesis is that God has done the very best He can, given the true counterfactuals of creaturely freedom which confront Him...God doesn’t create such a choice for Himself. The counterfactuals of creaturely freedom which confront Him are outside His control. He has to play with the hand He has been dealt.
But in that event, there's no justification for assuming that the card deck God has to work with includes a hand containing a feasible world in which no unreached person would be receptive to the Gospel if given the opportunity.
iv) By the same token, that's in competition with another one of Craig's conjectures:
Maybe His desire to achieve an optimal balance between saved and lost overrides the benefits of a world with less natural and moral evil.
But that means God must be dealt two royal flushes in a row. He must be lucky enough to have a feasible world which combines both an optimal balance between the saved and the lost as well as where no unreached person would be receptive to the Gospel if given the opportunity. But on his own grounds, Craig has no warrant for believing that the card deck includes a feasible hand where both rosy scenarios coincide.
5. C. S. Lewis famously said: "But the truth is God has not told us what His arrangements about the other [unreached] people are. We do know that no man can be saved except through Christ; we do not know that only those who know Him can be saved through Him.
That, of course, is very different from saying no one can be saved except through faith in Christ.
6. There's internal pressure in freewill theism towards inclusivism because freewill theists typically subscribe to universal atonement and God's ardent desire to save everyone. But that's in tension with belief that salvation is contingent on a condition which is unavailable to many people: knowledge of the Gospel.
Like it or not, Calvinism doesn't suffer from that internal pressure. Its commitment to exclusivism is internally consistent, given reprobation and limited atonement.
Offhand, the only Reformed theologian I'm aware of who embraced a "wider hope" is William Shedd. He's fairly idiosyncratic. His position is likely colored by his Christian platonism.
Let's consider some wedge issues:
7. OT saints
i) OT Jews didn't need to exercise explicit faith in Jesus to be saved.
True, but that means the content of saving faith is indexed to progressive revelation. To whom much is given, much is required.
ii) From the standpoint of pre-Christian Jews, there's a distinction between believing in the Messiah and believing in Jesus. They didn't know who the Messiah would be, but they knew what the Messiah would be. From our retrospective standpoint, we know that Jesus and the Messiah are one in the same person. From their prospective standpoint, they couldn't know that Jesus would be the Messiah. They didn't know about the life of Jesus. But they could still believe in the Messiah.
It's like saying you can believe in Superman without believing in Clark Kent. If you don't know that Clark Kent is Superman, that doesn't prevent you from believing in Superman.
To draw another distinction, you can know a role or know a character without knowing the actor who will play the role or play the character.
iii) According to the NT, and Hebrews in particular, there was a transitional phase where, once you know who the Messiah is (Jesus), it's no longer enough just to believe in the Messiah: you must believe that Jesus is the Messiah. At that point, rejecting Jesus is tantamount to rejecting the Messiah, since Jesus is the Messiah. You can no longer separate the two.
iv) An inclusivist might object that while there's a chronological distinction between Jews who lived before Jesus and Jews who lived after Jesus, that's analogous to a geographical distinction for gentiles who live outside the pale of the Gospel. They are in a position comparable to pre-Christian Jews. Even though they live after Jesus, they might as well be living before Jesus, because their geographical barrier is equivalent to a chronological barrier. Time and place are both buffers.
But a problem with that comparison is that you had the same geographical distinction in OT times. Yahweh revealed himself to Israel in a way that he didn't generally reveal himself to pagan nations. And to the extent that he made himself known to pagan nations, it was in connection with Israel. Post-Christian pagans are not in a situation analogous to pre-Christian Jews. Rather, they're in the same situation as pre-Christian pagans. God generally distinguished between Jews and Gentiles, except where their lives intersected.
8. Pagan saints
Some inclusivists classify some Biblical figures as "pagan saints": Enoch, Job, Noah, Melchizedek, Abimelech, Jethro, Naaman, the Queen of Sheba, Nebuchadnezzar, Ninevites, and Cornelius. But there are serious problems with that category:
i) Except for Cornelius, the Bible doesn't say they were saved.
ii) To the extent that some of them were saved, they came to saving knowledge through contact with the chosen people.
iii) Some of them were recipients of special revelation.
iv) Cornelius was a Godfearer. An intellectual convert to Judaism (although he eschewed circumcision). He's in the position of an OT saint.
v) How many inclusivists regard Job as a historical rather than fictional character?
vi) It's possible that Melchizedek was pagan. That doesn't make him a "pagan saint". That doesn't mean he was saved. His function is essentially symbolic. His typological role is separable from his person. What matters is what he represents, not his character.
Even Calvinists believe that "elect" infants dying in infancy are saved. So faith in Jesus is not a sine qua non for salvation. But there are serious problems with that comparison:
i) The argument either proves too much or too little. It's not just that babies lack faith in Jesus. They lack implicit faith. They lack faith in general revelation.
ii) Children below the age of reason lack the cognitive development to form propositional beliefs. That's not analogous to mentally competent agents. Rather, that's analogous to the developmentally disabled, or the senile.
iii) Although "elect" infants dying in infancy aren't saved by faith, they are saved by grace. They are saved by regeneration.
iv) But it might be objected that if that's the case, why can't other people be saved by grace or by regeneration rather than faith?
No doubt God could do so if he chose to. But mercy is discretionary rather than obligatory. Fact is, the Bible stresses the necessity of faith in Christ for salvation when addressing adults. Whether or not we find it arbitrary, that's our frame of reference. Even if there are exceptions, that is only known to God. We must operate by his revealed will, not his secret will–assuming God makes exceptions.
The OT records divine disclosure by theophanies and angelophanies, as well as dreams and visions. In the Bible, pagans are sometimes recipients of revelatory dreams. Some Christians identify certain OT angelophanies as Christophanies. Likewise, you have modern-day reports of Jesus appearing to Muslims in dreams, which are instrumental in their conversion.
If so, then in principle, why couldn't there be Christophanies to the unevangelized? To take one hypothetical scenario why couldn't Manitou sometimes be a Christophany to heathen Indians who had no access to the Gospel?
i) Even if that's hypothetically possible, unless we have evidence that it ever happens, so what?
ii) According to the Christian paradigm, faith in Jesus involves believing the gist of a biographical narrative about who Jesus is and what he did. By itself, a Christophany is not an object of faith. Even in the OT, event-media and word-media work in tandem.
iii) The OT presents the situation of the heathen as morally and spiritually dire. So does the NT (e.g. Rom 1:21-32; Eph 2:1-3,12; 4:17-19; Tit 3:3). Likewise, when Christian missionaries push into unevangelized lands, they encounter animism, paganism, and depravity. They don't encounter people-groups to whom God appeared in disguised Christophanies.
iv) If, moreover, God were to instigate a religious movement through a Christophany, without biblical revelation, that would rapidly degenerate into a pagan cult.
v) Now, for all we know, it's possible that God has appeared to select individuals throughout history, across the globe. But certainly not enough to establish a religious movement that allegorizes Christianity. At best, it could only be in reference to isolated cases.