History does not tell us much about Saint Thomas, also known as “doubting Thomas”. Yet, he gives us a powerful lesson on faith. The text which focuses on Thomas is; Then he said to Thomas, “Put your finger here, and see my hands; and put out your hand, and place it in my side; do not be faithless, but believing.” Thomas answered him, “My Lord and my God!” Jesus said to him, “Have you believed because you have seen me? Blessed are those who have not seen and yet believe.”
– John 20:27-29
His called “doubting Thomas” as he did not believe the words of the other Apostles regarding the Resurrection. Faith and doubt are opposites; faith is the belief in something not yet proven, whereas doubt has not yet arrived at a conclusion. The Catechism provides powerful teachings.
2088 The first commandment requires us to nourish and protect our faith with prudence and vigilance, and to reject everything that is opposed to it. There are various ways of sinning against faith:
Voluntary doubt about the faith disregards or refuses to hold as true what God has revealed and the Church proposes for belief. Involuntary doubt refers to hesitation in believing, difficulty in overcoming objections connected with the faith, or also anxiety aroused by its obscurity. If deliberately cultivated doubt can lead to spiritual blindness.
2089 Incredulity is the neglect of revealed truth or the willful refusal to assent to it. "Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him."
Doubt should not be confused with skepticism, which is the total rejection of a conclusion. As we see with Thomas, he never rejected the Resurrection- he simply wanted proof. The nail marks served as his proof.
Though known as “doubting Thomas”, early Church traditions connect him to Mary. The Girdle of Thomas, Virgin's Girdle, Holy Belt, or Sacra Cintola in modern Italian, is a Christian relic in the form of a "girdle" or knotted textile cord used as a belt, that according to a medieval legend was dropped by the Virgin Mary from the sky to Saint Thomas the Apostle at or around the time of the Assumption of Mary to heaven. The supposed original girdle is a relic belonging to Prato Cathedral in Tuscany, Italy and its veneration has been regarded as especially helpful for pregnant women. The story was frequently depicted in the art of Florence and the whole of Tuscany, and the keeping and display of the relic at Prato generated commissions for several important artists of the early Italian Renaissance. The Prato relic has outlasted several rivals in Catholic hands, and is the Catholic equivalent of the various relics held by Eastern Christianity: the Cincture of the Theotokos of the Eastern Orthodox Church and the Holy Girdle of the Syriac Orthodox Church. The legend of Thomas's girdle probably originated in the East, and was well known in Italy by the 14th century. Thomas is most famous, apart from his mission to India, for the Doubting Thomas episode (John 20:24–29) where he missed the post-Resurrection appearance of Jesus to the ten other apostles, and said he would not believe Jesus had returned until he had felt his wounds. In the story of the girdle, at the Assumption of Mary, where the other apostles were present, Thomas once again missed the occasion (being on his way back from India), so the Virgin Mary, aware of Thomas' skeptical nature, appeared to him individually and dropped the girdle she was wearing down onto him, to give him a physical proof of what he had seen. In other versions he was miraculously transported from India to the Mount of Olives, to be present at the actual Assumption, and the Virgin dropped her girdle down to him as she was taken up to heaven. Alternatively, only Thomas actually witnessed the Assumption, and the Virgin left the belt as a proof for his story to the other apostles (a neat inversion of the Doubting Thomas episode).
Therefore, although subordinated to other parts of our faith, Thomas has much to teach us. From him we see the difference between AGNOSTICS, those who, like Thomas, are open to God, and the SKEPTICS, for whom no amount of evidence is enough to convince them.