John Calvin on Deuteronomy 17:15

15. Thou shalt in any wise set him king over thee. First of all, God maintains His own supremacy in the appointment of a king, and does not consign the matter to the people’s own suffrages; that thus He may chastise their audacity in demanding a king in accordance with a hasty impulse. Secondly, He commands that he should be taken from the people themselves, and excludes foreigners, because, if they had been admitted, a door was opened to apostasy; for each would have tried to force upon them his native gods, and true religion would have been persecuted by the force and threatenings of the royal power. Behold why God would not suffer a king to be sought elsewhere but from the bosom of His Church; in order that he might cherish and maintain that pure worship which he had imbibed from his childhood.