In his inaugural lecture at the University of Franeker in 1675, Herman Witsius (1636-1708) spoke on the character of a true theologian (De Vero Theologo). Below is his definition of a theologian:
“By a theologian, I mean one who, imbued with a substantial knowledge of divine things derived from the teaching of God Himself, declares and extols, not in words only, but by the whole course of his life, the wonderful excellencies of God and thus lives entirely for His glory. Such were in former days the holy patriarchs, such the divinely inspired prophets, such the apostolic teachers of the whole world, such some of those whom we denominate fathers, the widely resplendent luminaries of the primitive Church. The knowledge of these men did not lie in the wiredrawn subtleties of curious questions, but in the devout contemplation of God and His Christ. Their plain and chaste mode of teaching did not soothe itching ears but, impressing upon the mind an exact representation of sacred things, inflamed the soul with their love, while their praiseworthy innocence of behaviour, in harmony with their profession and unimpeached by their enemies, supported their teaching by an evidence that was irresistible, and formed a clear proof of their having familiar intercourse with the most holy God.”
This lecture can be found in his Miscellaneorum Sacrorum Libri IV, and, should you be interested, an English translation is available here.