Doubting Thomas

A person who is very hard to convince is referred to as a "doubting Thomas." Among the disciples of Jesus Christ was one man who refused to believe in Christ resurrection, that disciple was Thomas.  Because of his doubts, he was called "the doubting or very Thomas." 

Since then, a person who doubts and hesitates unnecessarily on maters about which others have no doubts is said to be a "doubting Thomas" or "very Thomas."