Doubter

n.

1.

One who doubts; one whose opinion is unsettled; one who scruples.

Noun	1.	doubter - someone who habitually doubts accepted beliefs Synonyms: sceptic, skeptic
	2.	doubter - a person who doubts truth of religion Synonyms: agnostic

doubt, doubting Thomas, intellect, intellectual, pessimist, sceptic, skeptic