John 7:15 - Ellicott's Commentary On The Whole Bible

How knoweth this man letters? — Their spirit is seen in that at which they marvel. It is not the substance of His teaching that excites their attention, but the fact that He who has never been technically trained as a Rabbi is acquainted with the literature of the schools. (See Acts 26:24, “much learning,” where “learning” represents the word here rendered “letters.”) He is to them as a layman and unlearned (comp. Note on Acts 4:13), not known in the circles of the professional expounders — a demagogue, who deceived the multitude; and they hear Him speaking with a learning and wisdom that excites their wonder, and unlocking mysteries of which they thought that they only possessed the key.