Call Me a Skeptic...

While there isn't a single, perfect word in the English dictionary that exclusively means "someone who disbelieves everything authority says," there are several words that get incredibly close, depending on the exact flavour of distrust you are looking for.

Here are the best matches, ranging from intellectual skepticism to outright defiance:

The Best Matches

• Antiauthoritarian (noun/adjective)
  This is the most direct literal match. An antiauthoritarian is someone who is opposed to authoritarianism and strictly rejects the principles of strict obedience to authority.

• Contrarian (noun)
  A person who take up a contrary position, especially a position that is diametrically opposed to the majority or established authority. If authority says "A," a contrarian's default reflex is to believe "B."

• Nullifidian (noun)
  A rare and beautiful dictionary word. It comes from the Latin nullus (none) and fides (faith). It describes a person who has absolutely no faith or belief in any established system, doctrine, or authority.

• Iconoclast (noun)
  Historically, this meant someone who literally destroyed religious images. Today, it describes someone who aggressively attacks or rejects cherished beliefs, traditional institutions, and established authorities.

Subtle Differences in Flavor

If none of those hit the nail on the head, it might be because your definition leans closer to one of these three categories:

Word	Core Motivation	The Vibe
Skeptic / Pyrrhonist	Wants absolute empirical proof before believing anything.	Intellectual doubt.
Cynic	Believes authority figures are inherently motivated by self-interest and dishonesty.	Disillusioned distrust.
Maverick / Dissident	Chooses to think and act independently, actively opposing official policy.	Defiant independence.

A note on "Maverick": While we often use it today to mean a cool, independent thinker, it originally referred to unbranded cattle. It perfectly captures someone who refuses to carry the "stamp" or brand of institutional authority.