According to Muslim tradition, the angel Gabriel appeared to Muhammad from time to time to give him revelations. For Christians, that raises the question: What really happened to Muhammad?
Short answer: I don't know. I know what didn't happen to him. I know he didn't have an audience with the angel Gabriel. But barring that, what are the alternatives?
In principle, there are naturalistic and supernaturalistic explanations. We can also distinguish between mental and extramental experiences.
i) An angel did, indeed, appear to Muhammad. But of course, some angels are fallen angels.
ii) Arguably, not all evil spirits are demonic. Ghosts are a well-attested phenomenon. What if the souls of the damned sometimes appear to the living? That may be what happens during some seances.
For all we know, Muhammad dabbled in necromancy.
iii) He was possessed. I presume that's the most popular explanation among Christians. It can't be proven or disproven in Muhammad's case.
At his trial (according to Plato's Apology), Socrates talked about a "demon" (daimonion) that used to give him guidance. Of course, he didn't mean "demon" in the Christian sense, but he may have spoken better than he knew. Perhaps Muhammad's case was similar.
iv) He was psychotic. Suffered from hallucinations. That might be a naturalistic explanation.
On the other hand, possession and psychosis are not mutually exclusive.
v) William Blake was a visionary. As I recall, Kenneth Clark attributed his "visions" to Blake's eidetic memory. That's a naturalistic explanation. Might apply to Muhammad, although that's not the first explanation I'd reach for.
vi) He was a charlatan, like Joseph Smith. He made it all up.
That's entirely possible. There's certainly evidence, even in Muslim tradition, that he sometimes improvised.
We can't say for sure because we don't have as much information about Muhammad as we have about other cult leaders like Swedenborg, Joseph Smith, Sun Myung Moon, Herbert W. Armstrong, or Ron Hubbard–to name a few
In the case of Smith, Hubbard, and Moon, a naturalistic explanation is preferable.
In the case of Swedenborg, it may be more than that. Unlike Smith, who was a social climber, and had much to gain by conning suckers, Swedenborg came from the upper crust. He was a noted scientist. At the same time, he inherited his father's esoteric theology.
In his case, I tend to think something weird really did happen to him which could either have a naturalistic or supernaturalistic explanation. Psychosis. Possession. Perhaps he dabbled in the occult. Or maybe he suffered from mental illness.
There's the same range of diagnostic possibilities for Muhammad. Our information about Muhammad is one-sided, although it includes hostile testimony.