doctrina

Doctrina, doctrinae, f., first declension, from doceo by way of doctor, "one who teaches". Note that studeo takes the dative, so doctrinae studere, not an accusative.

Classically it names the activity of teaching or its result in a person: instruction, training, and hence the learning someone carries. It is standardly set against natura or ingenium in the old argument about whether talent or schooling makes the man. The ecclesiastical corpus narrows it hard, to doctrine in the English sense - a defined body of belief you can be orthodox or heretical about - and importing that back into Cicero will make him sound like a bishop. Doctus, the participle, simply means "learned".