De Bello Gothico (History of the Wars, Books V-VIII)

De Bello Gothico, Procopius of Caesarea's sixth century history of the Gothic Wars, read here in the standard Harvard/Loeb translation, contains in Book VII the earliest surviving description of early Slavic religion, including a supreme deity credited with sending lightning. Its strength is that Procopius was a Byzantine court historian with real access to military and diplomatic circles engaging Slavic peoples directly. Its limit is that he wrote as an outside observer, so his account reflects Roman categories of description as much as Slavic self understanding; no single year is asserted here because it is the passage's content the atlas draws on, not one dated event.