As far as the forces of Heaven go, Islam began with Yves. It started in the 7th century AD, amidst a climate of dissatisfaction with Judaism (too obscure) and Christianity (corrupted by politics). Many angels had come to believe that Heaven should try its hand at crafting something like Christianity from scratch, applying more guidance in its early phases.

Whatever his own reasons, the Archangel of Destiny handed the Qu'ran to Gabriel, with instructions to recite the sacred text to Muhammad. (He has yet to state whether the material is his own work, or something he took from one of his alleged chats with God, the existence of which he also doesn't clearly affirm or deny.) Gabriel, at the time at least tenuously sane and in fact known as a loyal and dependable messenger, did as he was instructed...although some would later claim that he took certain liberties in the process.

Islam's spread trumped even Christianity's heretofore unparalleled expansion, eradicating pagan support in much of the Middle East (and northern Africa). Hell attempted to counter the effort with the same tools it had employed against Christianity, and despite some success found it difficult to turn the tide. Some factionalism creeped into Islam, but it could not be destroyed. Now, the main question was whether or not Heaven liked what it had produced. Angels were dispatched to investigate the religion in more depth, as part of an assessment as to its long-term viability and desirability. One of these angels was Khalid, then Angel of Faith, a servant of Uriel. Although sent with a critically-minded mission from a pro-Christian Superior skeptical of the new religion, Khalid found Islam thoroughly Divine in theory and function, and would later become its patron Archangel.

Dominic, deeply involved in the Christianity project, was furious at the creation of another up-and-coming monotheistic religion. He checked the copies of the sacred text circulating among the Prophet's followers, compared them with the copy from Yves' library, and found discrepancies. Despite others pointing out that humans are not the most reliable when it comes to such matters, Dominic believed there was a more sinister explanation: the messenger had taken liberties with the divine message. He began to clamor for a trial, accusing Gabriel of heresy in a spectacle that got all of Heaven abuzz. While Dominic often comes across as the "bad guy" here, in his defense it should be said that the arising of another proselytising monotheistic religion did create quite a bit of havoc on Earth. (And at one point, angels nearly came to blows as members of opposing armies!) Furthermore, it's not unreasonable that his honest hunch may have been that Gabriel was to blame...and who knows, perhaps he's right.

Right or wrong, Dominic and his allies found their efforts swiftly opposed. Michael, himself having been subjected to such proceedings, loudly protested from the start that this was another divisive political move. A handful of others -- Janus and Eli in particular -- also questioned the idea of starting a trial and vouched for Gabriel's character. Finally, Yves defended Gabriel as well, albeit too mildly to fully exonerate him: he said he believed Gabriel was well-intentioned and trustworthy, and doubted he would do anything to sabotage the new religious project. The rest of Heaven's Archangels kept quiet, watching Gabriel with suspicious frowns. After all, he was under a lot of pressure lately...who could say?

Gabriel had long been sharing the Word of Fire with her Fallen servitor, Belial, and it took Archangelic willpower to not go completely insane under such a diabolical influence. (Belial by this point already had a summer home in Crazyville.) The trauma of being accused before Heaven as a traitor seemed to finally make him stop trying to stay sane. He turned his back on Heaven and retreated to Earth. This was the start of the lunatic Gabriel we all know and love.

Khalid, Archangel of Faith, is Islam's main proponent amongst the Archangels. (Zadkiel, Archangel of Protection, is also a supporter of Islam, though a more pragmatic one.) Like Christianity, it has a checkered record, and both sides have been able to use parts of its organization (particularly in times past) for their own ends. But, like the other Abrahamic religions, it's a major contributor of Essence to Heaven.