Except Voltaire never said that.

The statement "To learn who rules over you, simply find out who you are not allowed to criticize" is often falsely attributed to Voltaire, the French Enlightenment philosopher. However, the phrase is believed to originate from an essay by Kevin Strom first published in 1993: "All America Must Know the Terror that is Upon Us". He wrote: "To determine the true rulers of any society, all you must do is ask yourself this question: Who is it that I am not permitted to criticize?" I was in college in 1984 we were taught in Philosophy class that Voltaire said that. Revised history is what you are quoting.