Keraunobolon

Τάγμα ἦν τῷ Μάρκῳ (καλοῦσι δὲ τὸ τάγμα οἱ Ῥωμαῖοι λεγεῶνα) τῶν ἀπὸ Μελιτηνῆς στρατιωτῶν· εἰσὶ δὲ τὸν Χριστὸν πρεσβεύοντες ἅπαντες. ἐν οὖν τῇ μάχῃ ἐκείνῃ προσιόντα τῷ Μάρκῳ τὸν ἔπαρχον, ἀμηχανοῦντι πρὸς τὴν περίστασιν καὶ δεδιότι περὶ σύμπαντι τῷ στρατῷ, εἰπεῖν λέγεται ὡς οἱ καλούμενοι Χριστιανοὶ οὐκ ἔστιν ὅ τι οὐ δύνανται ταῖς εὐχαῖς, καὶ ὅτι παρὰ σφίσι τάγμα ὅλον τυγχάνει ὂν τούτου τοῦ γένους. τὸν οὖν Μάρκον ἀκούσαντα παρακλήσει χρήσασθαι πρὸς αὐτοὺς ὡς ἂν εὔξωνται τῷ σφετέρῳ θεῷ, εὐξαμένων δὲ αὐτῶν παραχρῆμα ἐπακούσαντα τὸν θεὸν τοὺς μὲν πολεμίους κεραυνῷ βαλεῖν, τοὺς δὲ Ῥωμαίους ὄμβρῳ παραμυθήσασθαι· ἐφ’ οἷς καταπλαγέντα τὸν Μάρκον ἰσχυρῶς τούς τε Χριστιανοὺς κατὰ δόγμα τιμῆσαι καὶ τὴν λεγεῶνα κεραυνοβόλον προσαγορεῦσαι.
(Xiphilinus, Epitome Dionis 71.9.3-5)

Marcus had a division of soldiers (the Romans call a division a legion) from Melitene; and these people are all worshippers of Christ. Now it is stated that in this battle, when Marcus found himself at a loss what to do in the circumstances and feared for his whole army, the prefect approached him and told him that those who are called Christians can accomplish anything whatever by their prayers and that in the army there chanced to a whole division of this sect. Marcus on hearing this appealed to them to pray to their God; and when they had prayed, their God immediately gave ear and smote the enemy with a thunderbolt and comforted the Romans with a shower of rain. Marcus was greatly astonished at this and not only honored the Christians by an official decree but also named the legion the ‘thundering’ legion. (tr. Earnest Cary)