Volumes have been written giving detailed analyses of the extraordinary things that occurred in the first thousand years of church history, events that influenced everything that came after them. In this brief overview, I’m going to look at five dimensions of activity that had monumental impact for the future history of Christianity.
The first such matter was the rise of the so-called “mono-episcopacy.” By the end of the first century, it was seen that the bishop of Rome had grown exceedingly more influential than other bishops of that period. Within the next century or so, the authority and power of the bishop of Rome was consolidated for all future history of the Roman Catholic Church. The singular authority that became located in the bishop of Rome gave the church a unifying base. The influence of the pope in the first thousand years of the church is almost impossible to measure.
In that light, we see the second major impact come to the fore — the innovations brought to Christianity by perhaps the most important pope of the first millennium: Gregory the Great. In his activities he consolidated the power vested in the sacraments of the church and spawned the vast sacerdotal system (priests through ordination receiving the ability to act as mediators of God’s grace to man through the sacraments) with which all future Catholicism would be associated.
A third element that had great influence on the future of Christianity was the rise of the monastic movement. Beginning with the extreme asceticism of people such as Anthony of the Desert (ca. 251–356), this radical brand of self-denial became institutionalized with the rise of various monastic orders, most of which exist to this day. These orders include the Benedictines, the Augustinians, the Franciscans, and others that date back several centuries.
Taken from "Setting the Stage: The First Millennium" by Ligonier Ministries (used by permission).