Gregory the Illuminator went to Armenia around 230AD and converted the king. Armenia kept its Christian identity through the centuries. There is an earlier tradition that the Apostle Thaddeus went to Armenia but we do not know that for certain. Christians went very early into Russia. There is a strong Orthodox Church there still today.
The gospel spread as far as Britain before the first apostles died. The Germanic tribes were all reached before the break up of the Roman Empire. At the Nicaea Council in 325AD, 800 bishops from the West and 1000 bishops from the East were present, including one signature from “John the Persian, of the churches of the whole of Persia and in the great India.”.
The significance of this is that the theology of the early church councils is not Western theology. No one church was in charge. They met as independent bishops in one fellowship of the saints. Reformed Theology is not Western. It is derived from the early church councils before the Roman church became corrupt and before that from Pauline theology and the theology of the first church council in Acts 15.
Augustine brought a partial reformation to the church in the 4th and 5th Centuries, but he was too influenced by the asceticism of Tertullian and Origen to do much. Augustine was able to defeat Pelagius, who claimed that man was not born in sin and could be saved by works, but he did not overcome the syncretism of Greek philosophy in the early church.