Charles Martel was the ruler of the Frankish people in the early eighth century C.E. He is most famous for defending the realms of Christendom from Islamic invasion in 732 C.E. when he won the Battle of Tours. Islamic armies never reached further north than Spain in Western Europe. Without Martel's victory, it is very possible that all of Europe would eventually have been Islamic instead of Christianity, and the whole history of the world would be very different.

Lay investiture is the name given to the common medieval practice whereby the Holy Roman Emperor, or another secular ruler, would appoint church officials without the consent of the Papacy. Gregory VII saw this as a challenge to his rule and banned the practice. He also subsequently excommunicated Emperor Henry IV when he ignored this rule. 

The schism between Eastern and Western Christianity had been developing for several decades before it was finally formalized in 1054 (the eleventh century) with the excommunication of the Patriarch of Constantinople by Pope Leo IX. From this point on, Christianity was divided between a Catholic West and an Orthodox East. The main cause of the schism was the centralizing and autocratic power of the Papacy in the eleventh century, which was challenged by the imperial power of Constantinople. This was the first of many subsequent divisions of Christianity that have occurred in the centuries since.

Simony is the practice of selling (or buying) positions in the Catholic Church. Although technically forbidden under church canon law, it was nonetheless extremely common throughout the early medieval period. It was an important part of various controversies throughout the Medieval period and contributed to the loss of faith, among certain parts of the European population, in the Catholic Church.

The Spanish Inquisition was founded in 1478 by the significant Spanish monarchs Ferdinand and Isabella. It was originally intended to judge the legitimacy of those Jews and Muslims who rather than choosing to flee Spain (both groups were banished from the country) instead opted to convert to Christianity. The Inquisition was set up to determine if these conversions were false or not. Later, the Inquisition morphed into a device to wield state power and ensure uniformity of religion through torture and execution. Contrary to popular belief, it was rarely used within Spain to torture Protestants simply because there were so few Protestants in Spain. The Spanish Inquisition would last, in some form or another, until the early nineteenth century.

During the medieval era the Eastern Orthodox Church was centered in the Byzantine Empire. The Patriarch of Constantinople was considered the leader of Eastern Orthodoxy. Constantinople was the capital of the Byzantine Empire.

Pope Gregory I served as Pope from 590 C.E. to 604 C.E. He is one of the most significant and revered Popes in the history of the Catholic Church. His writings, of which there were many, continue to be influential to this day. He is considered by most scholars to be the founder of the medieval Papacy and the man who ensured the continued power of the Pope, by transferring his spiritual authority into a more secular role. He was the first Pope to take an active role in the political life of Europe and laid the foundations for the behavior of countless Popes that followed.

The Great Schism of Christianity, sometimes called the East-West Schism, took place in 1054 C.E. It involved the fracture of Christianity between the Catholic and Eastern Orthodox churches. The dispute arose over different interpretations of scripture between two of the major centers of Christian theology - the Vatican and Constantinople. After the Great Schism Catholicism continued to be centered in Vatican City and Eastern Orthodoxy was centered in Constantinople.

The early Christian "apologetics" were theologians who were primarily concerned with proving the veracity of the Christian faith and defending Christian truth from outside attacks. Many of the earliest works of Christian theology fall under the category of "apologetics." Paul the Apostle and Augustine of Hippo are two famous examples of Christian "apologetics."