He was determined not to believe. All of the nonsense spread by some of the most unreliable people was now being repeated by some who had become his closest friends. All of this happening in a time of grief was just too much. Dead people stay dead. (There was the case of Lazarus not too far in the distant past but that one was because of the prophet Jesus.)
This member of the inner circle of Jesus’ followers would have known of any plot to steal a body. There was none. His fellow members of that inner circle were now trying to get him to believe that they had seen Jesus alive. They reported that they were in their locked hiding place and that Jesus came and stood among them and talked with them. Thomas was not going to accept it.
He heard the stories of the experiences at the tomb by the women who had not expected to see Jesus alive but saw him apparently well. He was told of the two followers of Jesus who did not recognize him until after talking with them about how God had planned all of the events of Jesus life and death. Then these two finally recognized Jesus as he repeated his custom of giving his thanks as he broke bread at the meal he was served.
Thomas was not going to be turned, even in spite of all that anyone could say. He was not going to believe any of it unless Jesus stood before him and Thomas was able to stick his fingers in the holes at Jesus’ hands and stick his hand in the hole in Jesus’ side.
That next week, Thomas got the invitation to do just that from Jesus himself.