The following passage from the Gospel of John will answer that question for us.
But Thomas, one of the twelve, called Didymus, was not with them when Jesus came. The other disciples therefore said unto him, We have seen the Lord. But he said unto them, Except I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe. And after eight days again his disciples were within, and Thomas with them: then came Jesus, the doors being shut, and stood in the midst, and said, Peace be unto you. Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing. And Thomas answered and said unto him, MY LORD AND MY GOD. Jesus saith unto him, Thomas, because thou hast seen me, thou hast BELIEVED: blessed are they that have not seen, and yet have believed. (John 20:24-29)
As you can see here, Thomas did not believe the testimony from the other disciples, when they told him that Jesus Christ was in fact alive. Thomas told them that he wanted living proof. Thomas wanted to put his fingers into the holes in his hands, and put his hand into the side which was pierced with the sword. Nothing else would be sufficient for this man Thomas. But you want to know something. The Lord heard it, and arrived eight days later to confront Thomas with his unbelief. The Lord asked Thomas to put his finger into the holes in his hand, and the hand into his side. Thomas was no longer a man of doubt after that. Thomas had seen the risen Jesus Christ with his own eyes, and that is when he referred to Jesus Christ as ‘my Lord and my God.’