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.

Thomas Aquinas was part of a growing scholastic movement in late medieval Christianity, meaning he believed that knowledge was important and reason could coexist with faith. Ideas about the duality of Christ and the cosmos come from Nestorianism and Manichaeism respectively, both of which are much earlier Christian belief systems. Justification through faith alone is a tenet of the Protestant Reformation, which came centuries after Aquinas.

Although early Protestant sects existed in the late medieval period, this new form of Christianity was not widely practiced until the sixteenth century. Martin Luther's 95 Theses, posted in 1517, marked the beginning of what came to be known as the Protestant Reformation.

The Abbasid Caliphate was founded by descendants of the Prophet Muhammad's uncle, al-Abbas and overthrew the Umayyad Caliphate.

The Five Pillars of Islam are Shahada, Salat, Zakat, Sawm, Hajj. The Shahada is the profession of faith all Muslims must make that "There is no god but God (and) Muhammad is the messenger of God." It is mandatory that one must recite the Shahada to become Muslim. Salat is the daily prayer that Muslims must make five times a day in the direction of Mecca. Salat is traditionally performed at dawn, noon, afternoon, evening, and night though it may be modified based on one's geographic location. Zakat is alms-giving or charity. Zakat is expected of those who have surplus to give, but not of those who do not. Sawm is fasting during Ramadan, the ninth month of the Islamic calendar. One cannot eat between dawn and dusk during the fasting period. Hajj is the pilgrimage to Mecca that all Muslim who have the means to do so are expected to make once in their life.

Muhammad began to receive the message of Allah through the archangel Gabriel in 610 C.E. Muhammad would go to meditate on the mountain near Mecca called Jabal an-Nour in a cave known as Hira. Muhammad began to speak publicly about the messages transmitted to him by Gabriel, signaling the beginning of the Islamic faith.

The Qur'an is divided into 114 surahs. The surahs are not listed in chronological order in the Qur'an as they were revealed to Muhammad. The first 24 surahs are Medinan, or revealed to Muhammad in Medina. These surahs are generally longer deal with concrete aspects of Muslim life. The other 86 surahs are known as the Meccan surahs. They are often shorter and deal with religious and spiritual aspects of the Muslim faith.

The hadith are important in the Islamic faith as they are reported to be the spoken word of Muhammad. The hadith can be used in matters of jurisprudence, meaning they can be used to form Islamic Holy Law. Many hadith are contested as to their veracity and hadith can only be confirmed with multiple accounts attributing the words being spoken by Muhammad.