What Christian Church did most people belong to in the Middle Ages?

The most important Church of the Middle Ages was the Catholic Church. When it split in 1054, it became the Roman Catholic Church in the West and the Eastern Orthodox Church in the East.

There were always other Churches. The Celtic Church, the Coptic Orthodox Church and the Oriental Orthodox Churches were very early and all predated the Middle Ages. The Celtic Christian Church was absorbed by the Catholic Church, but the others continue. There were also heterodox organizations outside orthodox Christianity, and some of these were declared heretical, in some cases resulting in military operations.