The Church was, from it’s very beginnings, a missionary organization. Jesus himself spent most of his time wandering the towns and villages of the Jordan Valley, Galilee and the Decapolis, teaching, preaching and making converts. After the Resurrection, He made his closest 11 disciples (Judas was no longer with them, remember) into Missionaries. That’s what ‘apostle” means in Greek: “one who is sent”. When He told them to “go into all nations, baptizing and making converts”, He was setting the standard for all future Christians. Now, it’s true that many branches of the Church, at many different times, had avoided that label, but just as many have accepted it gladly. The Christian Church is, first and foremost, a missionary enterprise.
That was a huge issue from about the fifth through about the fourteenth Centuries. Initially, even after the Council of Nicaea, it was not only not an issue, it was an almost universal truth. When Constantine legalized the Church, it almost immediately began to expand under the leadership of the first Patriarchs. Alexandria expanded south, into east Africa and the Arabian Peninsula. Antioch expanded eastward, through Persia and into India and even China and Japan. Constantinople went north, through the Caucasus into what we know as Russia and the country of the Slavs. Four of the five Patriarchates were fully immersed in missionary activity. In Rome, though, there were other priorities.
In Rome, it was a question of survival! All of Europe was being overrun by German tribes. The Vandals crossed the Rhine, took all of Gaul and Spain, jumped the Straits of Gibraltar and raced eastward along the coast of North Africa. Close behind them came the Goths and Ostrogoths, who settled in France and the Alps, and the Lombards (technically “longobarbs: long-beards) who settled in Northern Italy. Behind them came Angles, Saxons, Jutes (all of whom ended up in Britain, as well as Germany), and, worst of all, the Huns. The Patriarchs of Rome, the “Popes”, far from sending out missionaries to convert these barbarians, had them to contend with, right there at home. A further issue arose in that many of the Germans were already Christian converts, but they were of the Arian persuasion. Arianism, you’ll recall, was the first named Christian heresy, and with the Germans, was suddenly right next door to thousands of Catholic Christian Churches. The Roman Popes were hundreds of years behind their counterparts getting started on Missions outside their own native soil. At least part of the problem was that, with the Emperors being a thousand miles away in Greece, the Popes found themselves running much of the everyday civil government, as well as the Church. Barbarians are not very good at everyday government tasks like police and fire protection.