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 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.

Muhammad and his followers fled to the city of Medina in 622 C.E., a date which marks the beginning of the Muslim calendar. For this reason, Medina is very significant to the history of Islam, but Muhammad's home city of Mecca is the designated location of the Hajj, or pilgrimage.

The original source of the break between Shiite and Sunni Muslims was the line of succession to the caliphate, or leadership, of the Islamic Empire after Muhammad's death. While Sunnis hold Ali in high regard, they do not believe that the caliphate needed to follow his line of succession. Although the fall of the Umayyad Dynasty and its replacement with the Abbasid Dynasty was brought about by Shiites, this event did not initiate the Shiite/Sunni split. Both sides were in favor of expanding the empire.

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.

After the fall of the Roman Empire, the Roman Catholic Church became the most powerful institution in early medieval Europe. It was highly centralized, and therefore held a great deal of power over local rulers. The Catholic Church also mandated that services and prayers be performed only in Latin. On the other hand, the Orthodox Church of the Byzantines functioned as the official religion of a secular empire. It disagreed with the Roman Catholic Church on many points of theology, such as the Catholic concept of the Holy Trinity.

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.

The terms "Universal Church" and "Church Militant" express the formidable power that that Catholic Church attained during the Middle Ages, power which was best exemplified by the Crusades and the Inquisition. While Jews had always been persecuted by the Church, the Inquisition that began in the thirteenth century was more concerned with Christian dissenters. Christianity was the majority religion in Europe, but not in other parts of the world, such as the Holy Land that the crusaders failed to conquer.

Human sacrifice was crucial to Aztec religion, and continued military success allowed the Aztecs to capture the thousands of people killed in religious ceremonies every year. Otherwise, the Aztec Empire did not play a very hands-on role in the areas it conquered.

All of the belief systems given as options spread to China after the fall of the Han Dynasty, but Buddhism was by far the most influential. Neo-Confucians in the T'ang and Song dynasties mixed traditional Confucian ideas with Buddhist ideas the nature of the soul.