The western world is nearly destroyed by the onset of the Bubonic Plague. When the dust settles, the Muslims are poised to invade Europe, and do so. By the time of the Renaissance OTL, Europe is deeply Muslim, and the Pope has ceased having any power.
By the time of Columbus, the Muslims have launched a crusade against the Orthodox of the East, driving the Christians from the Caliphate of Thenas (Greece) who settled in Malorussia and the Orthodox Empire (Russia).
The Arab world by modern times is divided between the Shi'a, Sunni and Shim'ali sects, with the growing division of Atkellim Allah. The Orthodox Russian Empire is a last stalwart against the Muslims, and Rome is dominated by the great Mosque of Kephas, a great prophet following Jesua.
There was much strife between the sects of Islam in North America as they fought jihad against the infidel American Indians.
In the Caliphate of Al-Halam Aljideed there arose in the 1820's a new imam, splitting the Shia, and attracting many Sunni and Shim'ali to a new breed of Islam, called Atkellim Allah, or as it has now been shortened to Atk'Allah.