Faith--the belief that there is no god but Allah and Muhammad is his Prophet--is the missing pillar here, not marriage.

Abu Bakr succeeded Muhammad in leading the umma upon his death in 632 C.E. Abu Bakr was a political advisor to Muhammad and the first converted Muslim. However, as he was not related by blood to Muhammad, some contested his legitimacy. The Sunni faith grew out of recognizing Abu Bakr and the following three Caliphs of Umar, Uthman, and Ali as the Four Rightly Guided Caliphs. Sunnis believe that blood relations shouldn't determine the umma's leader, but that it should be determined by competence and strength.

Although the Catholic Church had certainly been the subject of controversy before the papal move to Avignon in 1309, the new practices and policies instituted by the Avignon papacy drew criticism on an entirely new level. Before very long, the papacy at Avignon became synonymous across Western Europe as a hotbed of political scheming and economic hypocrisy, even in the eyes of the some of the Church’s formerly loyal laypeople. Many clergymen had been utilizing their lofty social status as ripe opportunities for personal enrichment for many years, but this practice expanded in Avignon, opening up the Church to charges of hypocrisy and economic corruption. Other individuals assailed the judgments issued by the Curia, or the official papal court, as being unduly politically motivated, rather than religiously inspired, while others began to question the Church’s practice of collecting taxes, also known as annates, from a wide sector of essentially every social stratus. But perhaps the most controversial of all Church policies was the selling of indulgences - aka papal guarantees of alleged reprieve from punishment or time spent in Purgatory for the souls of already-deceased individuals. These indulgences, which were greatly expanded under Pope Clement V after the move to Avignon, were aimed at living Catholics who wanted to somehow spare their deceased loved ones from any suffering in the afterlife, which the Church promised it could prevent, provided that first a bill of sale for indulgence was purchased.

The Virgin birth is a core tenet of Christian faith. The others are all examples of practices that different factions within the Church debated.

The Knights Templar and the Knights Hospitaller were militant religious groups that fought for Christendom (and their own personal wealth) during the Crusades. Both groups became immensely rich and powerful following the conquest of Jerusalem in 1099, during the First Crusade. They continued to fight in European wars, particularly against the forces of Islam, for several centuries.

Any explanation of religious diffusion from 600 C.E. to 1450 C.E. should account for more than one faith and include some reference to religious warfare, which is exemplified by major events such as the Crusades and the spread of the Islamic Empire. Religious ideas did spread in other ways, but missionary work is a more widely relevant example than trade routes. Religious tolerance was not practiced by many of these nascent world religions, a reality that European explorers quickly made clear to New World civilizations. Finally, the Spanish Inquisition drove out most non-Christians as opposed to creating new devotees to the faith.

Jerusalem, which is also a holy city to Jewish and Christian people, is an Islamic holy city because it is believed that the Prophet Mohammed ascended to heaven from Jerusalem. The fact that Jerusalem is a holy city in the three largest monotheistic religions in the world has been a source of constant tension and bloodshed for more than one thousand years.

Judaism, Christianity, and Islam were the earliest religions to practice monotheism, or belief in a single god. While all three religions trace their origins to the prophet Abraham, only Jews consider him to be the founder of their faith. As members of the first monotheistic religion, Jews also see themselves as chosen ones, and do not place much emphasis on converting others to their faith. Neither Jews nor Muslims believe that Jesus was the son of God.

Islam arrived in Ghana in the ninth century and many individuals privately converted to the new faith. The state itself, however, did not adopt Islam as its official religion until forced to do so by the invading Almoravid Berbers. Mali, on the other hand, peacefully converted to Islam and benefitted immensely from their relationship with the Islamic world.

The division between Sunni and Shia Islam largely revolves around who should have been appointed Caliph of the early Muslim community.

Both Sunnis and Shias perceive Mohammed as a prophet.

There were political disagreements amongst the early Muslim community about the size of the empire, but this was not the split between Sunnis and Shias.

War with Rome was not the reason for the Sunni-Shia split.

While some contemporary Shia sects are said to believe in reincarnation (it's incredibly difficult to determine whether they do because these sects are officially closed to outsiders) the vast majority of both Shia and Sunni do not, and reincarnation was not the cause of the split between these two groups.