Franks were pagan when they joined the empire, and converted to Nicene Christianity. As JPierreD has said, the Burgundians, Visigoths, Ostrogoths, Vandals, Sueves and Lombards had a ruling elite that were Arian, though the populace of the kingdoms were largely Nicene. Unlike early struggles between Nicene and Arian theology, which struggled over control of the forming Roman Church, in these Kingdoms the Arians had their own church structure.