Ok, this quote is not about magic, but rather about story telling and the study of the classics. Nevertheless, I think that there is something to be learned from it.
Anyone who has recieved from God the gift of knowledge and true eloquence has a duty not to remain silent: rather should one be happy to reveal such talents. When a truly beneficial thing is heard by many people, it then enjoys its first blossom, but if it is widely praised its flowers are in full bloom. It was customary for the ancients, in the books which they wrote (Priscian testifies to this), to express themselves very obscurely so that those in later generations, who had to learn them, could provide a gloss for the text and put the finishing touches to their meaning. Men of learning were aware of this and their experience had taught them that the more time they spent studying texts the more subtle would be their understanding of them and they would be better able to avoid future mistakes. Anyone wishing to guard against vice should study intently and undertake a demanding task, whereby one can ward off and rid oneself of great suffering.