He was not called Doubting Thomas in the Bible. The nickname came about later. He was called that because of what happened after Jesus was resurrected. After Jesus was resurrected, he went to the house where the apostles were. Thomas was not there. When the others told Thomas that Jesus had appeared to them, Thomas said, “Unless I see the mark of the nails in his hands, and put my finger in the mark of the nails, and my hand in his side, I will not believe.” (NRSV, John 20:25)