Thomas, the one called Didymus, one of the Twelve, wasn't with the disciples when Jesus came.
References for John 20:24
The other disciples told him, "We've seen the Lord!" But he replied, "Unless I see the nail marks in his hands, put my finger in the wounds left by the nails, and put my hand into his side, I won't believe."
After eight days his disciples were again in a house and Thomas was with them. Even though the doors were locked, Jesus entered and stood among them. He said, "Peace be with you."
Then he said to Thomas, "Put your finger here. Look at my hands. Put your hand into my side. No more disbelief. Believe!"
Thomas responded to Jesus, "My Lord and my God!"
Jesus replied, "Do you believe because you see me? Happy are those who don't see and yet believe."
Then Jesus did many other miraculous signs in his disciples' presence, signs that aren't recorded in this scroll.
But these things are written so that you will believe that Jesus is the Christ, God's Son, and that believing, you will have life in his name.