The great journeys of evangelism recorded in the Book of Acts were undertaken by members of the Orthodox Church. The martyrs, those holy men and women whose zeal inspired others to seek God, were Orthodox Christians. The Roman Empire was converted by the Orthodox Church. The barbarians of Western Europe were converted largely by Orthodox Christian monks, often falsely associated as belonging to the “Roman Catholic” Church which came much later in history. When Western Europe was being evangelized, the Roman Pope was a bastion of Orthodoxy. Towards the end of a united Christendom, even the Slavs in the East of Europe were converted from Constantinople.
https://orthodoxreflections.com/the-bright-future-of-orthodox-missions-in-the-united-states/