Jesus died in our stead and arose, giving us victory over sin and death. When He rose from the dead, He showed himself to Mary Magdalene and the disciples. Thomas was not there when Jesus revealed himself to them. He found it difficult to believe that Jesus was alive. He said he would only believe if he could see the hole the nail had made and thrust his hand in his side.
It wasn't entirely out of place for Thomas to ask for hard evidence. He couldn't understand how a man who was crucified, bound up and buried in a sealed tomb could resurrect. It is the same with us sometimes. Based on our circumstances, we may find it hard to believe that the risen Christ exists.
When Jesus appeared and showed him his hands and side, Thomas proclaimed, 'My Lord and my God'. This is a statement of praise and worship to Jesus for dying on our behalf. It is also a statement of embarrassment in humility for not believing in His Saviour.