The noun skeptic means someone who doubts the truth or value of an idea or a belief.

Synonyms are cynic, dissenter, agnostic, or unbelieving.

The word origins from 1580s meaning “member of an ancient Greek school that doubted the possibility of real knowledge”. It comes from Middle French sceptique and directly from Latin scepticus meaning “the sect of the Skeptics”. It is derived from Greek skeptikos (plural Skeptikoi “the Skeptics, followers of Pyrrho”), noun use of adjective meaning “inquiring, reflective”.

Because I didn’t see more of the evidence, I became more of a skeptic.

They say it can cure serious cancer, but the doctor is a skeptic.

Do everything you can to convince the skeptics.