Definition
One who requires proof before believing.
One who finds belief difficult.
Origins
Originates from the story of the Apostle Thomas in the Gospel of John, where he refuses to believe that Jesus has risen from the dead unless he can see and touch the wounds of Jesus himself.
See Also
How People Actually Use It
A person who does not trust his/her best friend, even though the best friend has proven them self to be virtuous.
""My best friend won't trust me with his sister" "Dude, He is such a Doubting Thomas" "Yeah, I feel kinda Judased"
Source: Urban Dictionary