The Early and High Middle Ages were formative times for the Catholic Church. The fall of the Western Roman Empire in the late fifth century set the Church on a path that would see it as one of the few uniting forces in the region. Catholic leaders were presented with numerous barbarian peoples, some of whom were more receptive to Christianity than others. Missionary efforts led to the conversion of the Visigoths, Lombards, Franks, Anglo-Saxons, Vikings, and others. As a common faith among so many different and sometimes antagonistic peoples, the Catholic Church often found itself in the difficult spot of picking sides and being the peacemaker.

In the seventh century, the Byzantine prohibition of icons led to a major rift. This and several other ecclesiastical differences saw the Eastern Orthodox Church and the Catholic Churches take separate paths. At times, they would come to each other's aid. Other times, they acted more as antagonists. When Charlemagne invested the office of the Pope with the title Patricius Romanorum in 800, the Catholic Church became fully independent from Constantinople. Over the next two and a half centuries, the Eastern and Western churches maintained a strained relationship that culminated in a complete schism in 1054.

The partnership between the Catholic Church and secular leaders led to some challenges. For instance, the power to appoint members of the clergy became a recurring bone of contention. This led to the Investiture controversy of the eleventh and twelfth centuries. In short, this was a struggle between the Church and the Holy Roman Emperor over who had the power to appoint high-ranking clergy, including the Pope himself.

The latter half of the eleventh century saw an effort to rid the Church of corruption. Known as the Gregorian Reforms, initiated by Pope Gregory VII, these efforts reaffirmed papal authority and attacked practices of simony and clerical marriage.