The other disciples had already seen the prints of nails in Jesus’ side. Thus, they had no need to say the words of Thomas; but even seeing this, they still couldn’t reach a firm belief in the Resurrection.
The title “Lord” often articulated in the New Testament, and then in the first Christian centuries, refers particularly to Christ. It means the Supreme Lord, the attribute of the One Unique God. Seeing Him truly risen, Thomas recognizes Him as the One True God; One Who rules over all creation and is not overtaken by anything. He is the One God, the Master of all and therefore God of all, Who can not be conquered by death.
By crying, “My Lord and my God,” Thomas also expresses an emotional relationship of human love: “You are my Lord!” Thus, Thomas finds the ultimate foundation of his existence.
In general, man is seeking for the absolute, for a foundation that does not depend of mortal things. We’re not solely satisfied by a relationship with things that cannot grant us eternal life, cannot give us the infinite. There must be Someone Who can give sense to the human person and the things in the world we depend on, and in Whom everything finds meaning.
In Christ we were showed this very foundation and the intangible was made accessible to us. By saying “My Lord and my God,” Thomas declares that he has found the Absolute, that ultimate foundation for his life. Through His Resurrection, Christ had shown us that He can give life even to the weak flesh, and through His precious body, God is united with all men. He united our mortal body with the eternal.
(Father Dumitru Stăniloae, Commentary to St. Cyril of Alexandria’ homily on the Gospel of John)