'theologian' ultimately originates from Greek, specifically the word 'θεολόγος' (theologos), where 'theo-' meant 'god' and 'logos' meant 'speech, reason, or study'.

'θεολόγος' passed into Late Latin as 'theologus' and Old French/Medieval Latin forms (e.g. 'theologicus'), and through these medieval forms entered English as 'theologian' in Middle/early Modern English.

Initially it meant 'one who speaks about or studies God', and over time it came to mean more specifically 'a scholar or expert in the academic study of theology'.