From time to time, this question pops up in ex-Muslim cyberspace, sometimes composed as "Is Islam the truth?" Although the two questions are semantically different, they are meant to convey the same idea. But what idea, exactly?
"Is Islam true?" can be understood in at least two ways:
1. Are the claims made by Islam objectively true?
2. Is Islam a true religion?
Both subject Islam to scrutiny, or at the very least, cast doubt on it, and are intended to leave Islam either with or without merit, depending on whether the question is answered in the affirmative or the negative. Generally, either one or the other of these questions, or some conflation of the two, is intended when the question is posed, although this is seldom explicitly stated. Let us look at both.
Are the claims made by Islam objectively true?
Islam is many things, most important here is that it is a religion. What defines a religion is not its contents, but the relation of that religion's adherents to it. The adherents of a religion accept their religion's claims as true, the objective veracity of those claims not only being an inadmissible issue, but an offensive one. The religious believer believes. Belief requires that there be no proof. In the presence of proof, belief becomes superfluous as knowledge will have taken its place.
The act of believing belongs to faith, a very specific state of mind that cannot admit interrogation of any kind. It is not so much a matter of whether religious claims can or cannot be proven, they must not be proven, or more precisely, they must not be thought of in terms of proof. This means that even if a particular religious claim can be shown to be objectively true, that attribute would not only be irrelevant to a believer, it would constitute a threat. While the less sophisticated believer will be affirmed in his faith if his religious claim is proven to be true, the more sophisticated believer will see in such objective proof a degradation of the process of belief through its contamination by reason. The more reason worms its way into what the believer holds to be true, the more tightly his faith is hemmed in to an ever-shrinking domain within his cognitive world. To the believer, faith itself, rather than the specific religious claims believed in, is the good to be preserved and defended, and with every claim proven, especially if in favour of the religion, faith finds itself with less ground to stand on.
The defence of faith is the believer's answer to reason undermining his belief. It is therefore not possible to prove to a religious believer that the earth is a spheroid rotating on its axis and revolving around the sun, partly because the believer already knows this, and partly because the believer will shut out the implications of this knowledge. The believer will simply stop claiming that the earth is flat, or that the sun sets in a muddy spring (although there are notable exception, on which more later) and hold onto his faith without such claims.
But how does the believer deal with the knowledge that the sun does not set in a muddy spring? That depends on the extent to which the believer's religion allows reason (and ethics) to change it. Generally, religions have to be forced into admitting change. Put differently, only with the greatest reluctance does the believer inch back faith.
Most religions will move the particular, now rational, knowledge out of the domain of faith, and reformulate or reconceptualise faith, allowing belief to continue, e.g., "I don't believe in a personal god, but I still believe in a higher power," or "I don't believe in organised religion, but the spiritual dimension is important". Others will attempt to press irrefutable objective truths into service to bolster their faith, e.g., the dinosaurs fitted into the ark because they were ...baby dinosaurs! On a grander scale, the Pope prosecuted Galileo Galilei for having a telescope. Today the Vatican has its own observatory to gaze upon God's creation. Let us not get started on why Muhammad was not a paedophile. Muslims shoehorning a six-year-old girl to fit into the definition of a woman is bereft of all decency. Without losing sight of the appalling things that believers have done to heretics down the millennia, such changes coral, and through that enfeeble, faith to such an extent that believers are effectively weened off religion without actually becoming atheists.
All this notwithstanding, there is benefit to subjecting religious claims to rational scrutiny if the religion in question does not intrinsically preclude change. Those believers who depend more strongly on faith in order to be able to navigate life, will simply dig in their heels and insist that there were two people called Adam and Eve and that we all descend from them. They do this not because they know human evolution to be wrong, but because abandoning Adam and Eve undermines faith and they cannot live without faith. By holding the first line, belief, they can preserve their faith in its original form. Such people find themselves increasingly isolated from mainstream believers who have reconceptualised their faith and moved on.
Islam, by contrast, does not leave its purist believers in the lurch. Instead, in Islam, 'purist believers' is a tautology. Islam allows only purist believers, condemning as 'kafir', unbeliever, anyone who deviates from the religion as put forth in the Qur'an and the example of Muhammad's life. So-called scholarly consensus (ijmaa) pertains to interpretations of the religion's strict prescriptions. Islam has various built-in mechanisms that fortify it against change, from doctrinally stipulated social pressure all the way to killing.
A Muslim has far greater difficulty dealing with the sun not setting in a muddy spring than any other believer would. While the fundamentalist Christian might hold the line at the literal truth of every Biblical story, any suggestion that one Islamic claim might not be true amounts to an attack on the religion as a whole and an attempt to lure the Muslim away from Islam. To non-Muslims for whom faith is something to hold onto, the validity of belief as a mode of apprehension per se will not be drawn into question, though its reach may be rolled back where you start looking like a fool for clinging to it.
The question, "Is Islam true?" is of course directed at Muslims, and apart from anything else, a Muslim ceases to be a Muslim should he or she even consider the question. Trying to persuade a Muslim that Islam is not true, or worse, trying to debate the question with a Muslim, can only lead to frustration, as neither is possible. Should a vexed Muslim ask a "scholar" about irrefutable scientific proof that an Islamic claim is untrue, the scholar is extremely unlikely to engage with the claim, but rather will rather will, at best, urge the Muslim to strengthen his faith, or at worst, abuse the Muslim for doubting the Islamic claim. "Allah knows best" remains the scholars' ultimate get-out-of-jail card.
The best that can be hoped for is the Muslim laying out what he believes and, if the non-Muslim is very lucky, refraining from trying to humiliate him. But the veracity of the claim will not be engaged with because, apart from the cognitive issues discussed above, it is an impossible question for a Muslim to entertain. Indeed, it is a sin for him to entertain it. The question, "Is Islam true?" (are the claims that Islam makes true), only has meaning to those for whom belief is not an adequate mode of apprehending reality. This makes the question moot.
Is Islam a true religion?
Adherents of monotheistic religions are generally the ones most exercised by such a question. The seldom-stated axiom underlying the assertion that X is a true religion, or Y is not a true religion, is that a true religion supposedly emanates from "God". Conversely, if a religion does not emanate from God, then it is false. This axiom is, of course, premised on an even more fundamental axiom: that there is such a thing as God.
Neither of these axioms stand up to reason. Therefore, only one who submits the question to reason will see that either all religions are true or all religions are false, because without the axiomatic premise of religious believers, the question becomes, "Is X a religion?" I think we can accept the underlying question of whether or not there is such a thing as religion as answered in the affirmative. We can also accept that Islam, like Judaism, Christianity, etc., meets the criteria for being a religion, given that its relation to its adherents is mediated through faith.
The question of whether Islam is a "true religion" in the sense that it emanates from something for which there is no proof makes nonsense of the very idea of a "true faith". However, it may still be asserted and shown that Islam does or does not form part of a particular religious pedigree, e.g., the so-called "Abrahamic faiths". That, however, is very different to saying that a religion being Abrahamic makes it "true" and not being Abrahamic makes it "false". Either way, the idea presupposes that "the God of Abraham", as the phrase goes, is the "true god". Those who assert that Islam is not a true religion mean by that that Islam does not emanate from the God of Abraham and is therefore variously from a "false god", "from Satan" or from a "false prophet".
All religions are true only in the sense that they are all ritualisations of faith, but none of them are true if their veracity is premised on the provenance from some supernatural being or beings. So the question, "Is Islam a true religion?" in contradistinction to other religions presumed to be true religions, makes no sense. A believer, however, can never consider the question in these terms. It might be possible to persuade a Muslim that Islam is "false", but only in the sense that Allah in not the true God and the Muhammad is a "false prophet", as opposed to a supposed "true prophet". In such a case the Muslim will switch gods. This is possible because the question remains within the realm of faith. For the Muslim to understand that Islam is true in exactly the same way that, say, Christianity is true, and false in exactly the same way that Christianity is false, requires the Muslim to step outside of faith. And if this has already occurred, then "Is Islam true?" becomes superfluous in every sense. No debate will persuade him of this, for he will already be persuaded.
The problem of prophets
When we begin to step away from gods, thing get a little more interesting. Are the prophets true or false? Clearly, they can only be messengers of gods in the same conceptual way that the gods are gods. It is a role that can be believed in, but which can never be verified. This does not mean, however, that a prophetic role cannot be objectively ascribed to a person. Whether that person actually exists or existed is an altogether different question, and is indeed verifiable. Whether verified or not, believers will hold claims of prophethood to be true, while others will interrogate the historicity of the person to whom prophethood is ascribed.
Should the claims for the existence of a person described as a prophet be verified, then for believers, this will be enough to verify not only such person's prophethood, but everything about their religion. Disproving the existence of someone held to have been a prophet could pose an immediate existential crisis for the religion concerned, and either an identity crisis, or an existential crisis to that religion's adherents, depending on how interchangeable the god and the prophet are.
Not so with gods. The reason gods are safe is that claims for their existence are unverifiable one way or the other. Prophets, whose existence is conceptually grounded in objective reality, a real person, are not so lucky. This means that it is possible for someone to believe in God, yet question the existence of a prophet, especially one of another religion.
If a prophet's claimed historic existence is proven to be false, this still has no bearing on the question of whether their prophet was or was not a "false prophet", given that prophethood is held to be bestowed by a god who is unverifiable. It simply means that they got the wrong guy. The "true prophet" is still out there. Yet, this would be a concession too far for most believers to make. Christians might step back from Jesus' miracles, but they cannot abandon Jesus, as Jesus is part of the Holy Trinity that is God.
Muslims, who stand ready to kill for Muhammad, and are doctrinally bound to heed and emulate his every word and deed, reject those narrations of his words and deeds that embarrass them, and try to assert that such narrations as not authentic, even when they are classed as such. "Qur'an-only Muslims" go further and reject all hadiths, claiming that all that Muhammad had to teach is already in the Qur'an. This does not solve their problem, though, as the Qur'an, too, has come to embarrass them. Like the Christians who reject miracles but cannot reject Jesus, the Muslims who reject the hadith cannot reject the Qur'an, Allah's revelation to Muhammad. Instead, they end up entangled in the same cognitive contortions as those who try to sanitise the hadith.
To conclude, when secular people or atheists ask whether a religion is true or false, it is a sloppy reference to the claims of the religion that pertain to objective, textual, or historic reality. As with gods, where no claims are verifiable, the matter here too is clear-cut. This time, all claims are verifiable. A curious situation now arises. For believers, any verification of any religious claim as true is sufficient to confirms all claims as true, while proving any claim as false is insufficient to confirm any other claim as false. The believer will go silent about one refuted claim and simply put forward another, all to preserve their all-important faith. The Christian or Jew might find a happy compromise where they can still hold onto faith in some form or another. To the Muslim it is all or nothing. The bottom line is that the question, "Is Islam true?" only has meaning if the answer precedes the question, whatever that answer might be.