This is St. Thomas (a.k.a. "Didymus") inspecting the lance wound in Christ's side in order to overcome his doubt about Jesus' Resurrection. Hence the term "Doubting Thomas". From the Gospel of St. John the Evangelist (Chapter 20, verses 24 - 29): 20 24 Now Thomas, one of the twelve, who is called Didymus, was not with them when Jesus came. 20 25 The other disciples therefore said to him: We have seen the Lord. But he said to them: Except I shall see in his hands the print of the nails and put my finger into the place of the nails and put my hand into his side, I will not believe. 20 26 And after eight days, again his disciples were within, and Thomas with them. Jesus cometh, the doors being shut, and stood in the midst and said: Peace be to you. 20 27 Then he said to Thomas: Put in thy finger hither and see my hands. And bring hither the hand and put it into my side. And be not faithless, but believing. 20 28 Thomas answered and said to him: My Lord and my God. 20 29 Jesus saith to him: Because thou hast seen me, Thomas, thou hast believed: blessed are they that have not seen and have believed.
Jesus said to him, "Is it because you have seen me that you have believed? How blessed are those who have never seen me and yet have believed!"