Saint Thomas was called Didymus, which means 'the twin.' Someone asked me about this recently. Thomas did not have actually have a twin sibling. He was called 'the twin' because of his split personalities, if you will. He is a faithful apostle, yet he doubts.
When Jesus decides to see Lazarus, though it will mean traveling into the lion's den, Thomas says, "let us go that we may die with him" (John 11:16). When Jesus says at the Last Supper that he is going to the Father, Thomas asks what the way is, to which our Lord responds: "I am the way, the truth and the life" (John 14:15). When Jesus comes back to life, Thomas resists: "Unless I see the mark of the nails in his hands and put my finger into the nail marks and put my hand into his side, I will not believe" (John 20:25).