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.