Complex sentence: Somewhere in the east the Burgundians had Christianized to the Arian form of Christianity from their native Germanic polytheism, which proved a source of suspicion and distrust between the Burgundians and the Catholic Western Roman Empire.
Split into simpler sentences: Somewhere in the east the Burgundians had converted to the Arian form of Christianity from their native Germanic polytheism. Their Arianism proved a source of suspicion and distrust between the Burgundians and the Catholic Western Roman Empire.