"Doubting Thomas" is a phrase used to describe someone who is skeptical or hesitant to believe something without proof. Synonyms for this term include skeptic, cynic, disbeliever, quandary, and hesitant. A skeptic is someone who doubts the validity of something and requires evidence to believe in it. A cynic is a person who often believes that others have selfish motives and is skeptical of their actions. A disbeliever is someone who cannot conceive that something is true. When someone is in a quandary, they are uncertain or hesitant to make a decision. And a hesitant person is someone who is indecisive and might lack confidence in what they believe.