Incidentally, I stumbled on this while looking for a Marx quote about how representatives of the old feudal order falsely positioned themselves as allies to the lower classes by attacking the excesses of industrial capitalism. If anyone has that quote, please let me know.
Kings are the top of the Second Estate (the nobility or military caste). The priesthood being the First Estate is kind of a polite fiction.
It was the ethical and theological justification for the way things were.