^English form the 17th century (OED).
The Greek term is explicitly coined by Josephus and isn't attested elsewhere in Ancient Greek; Josephus marks it as a nonce coinage by calling it a "strained expression".
W. Whiston tr. Josephus, Against Apion ii. §17 (1814) IV. 340: "He [Moses] ordained our government to be what, by a strained expression, may be termed a Theocracy", translating ὡς δ'ἄν τίς εἴποι, βιασάμενος τὸν λόγον, θεοκρατίαν