The paper investigates the documentary value of Gregory of Nyssa's letters. The use of different literary forms may constitute the cause of and the reason for some distortion of the real, historical facts. There is proportionality between undecorated speech and poetical formulation on the one hand, and truth opposing the delightfulness as the main goal of the speech on the other. This proportionality cannot be exaggerated. Rhetorical devices are not simple formal tools in Gregory of Nyssa's practice which are sometimes employed for illusionist purposes and are sometimes neglected on the ground of a rational decision. They constitute a language that enables the writer to tell us the truth or his feelings, his intentions in a good style. Second Sophistique and Christianity form a unified culture in Gregory of Nyssa's oeuvre, despite the self-contradictions, ambiguities and difficulties, which were partly recognized by himself.