Saint Thomas the Apostle Freed the pagans from unbelief by proselytizing Jesus Christ

Matthew 28:19-20 

¹⁹ Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, ²⁰ teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age.” 

Saint JEROME (Catena Aurea)

 They first then teach all nations, and when taught dip them in water. For it may not be that the body receive the sacrament of Baptism, unless the soul first receive the truth of the Faith. In the name of the Father, the Son, and the Holy Ghost, that they whose Godhead is one should be conferred at once, to name this Trinity, being to name One God.

Saint JEROME (Catena Aurea)

Observe the order of these injunctions. He bids the Apostles first to teach all nations, then to wash them with the sacrament of faith, and after faith and baptism then to teach them what things they ought to observe; Teaching them to observe all things whatsoever I have commanded you.

Homily of St. Gregory, Pope on the Feast Day of St. Thomas the Apostle

What, dearest brethren do you notice in this passage? Do you think that it happened by chance, that this chosen disciple was absent at that time, and afterwards coming, heard the news, and hearing doubted, that doubting, he touched, and touching, he believed? This did not happen by chance, but by divine dispensation. For the divine clemency brought it about in a wonderful way that the doubting disciple, while touching the wounds in his master's flesh, should thereby heal the wounds of our unbelief. The unbelief of Thomas is more to our faith than the faith of the believing disciples. While he is brought back to faith by touching, our minds are set free from doubt and established in the faith.

So the Lord indeed after his resurrection permitted his disciple to doubt, but he did not leave him in unbelief; just as before his birth he wished Mary to have a spouse, who however never attained to the married state. The disciple who doubted and touched his risen Lord thus became a witness to the truth of the resurrection, just as the spouse of his mother was the guardian of her inviolate virginity. Thomas touched, and cried out: My Lord, and my God. Jesus said to him: Because you have seen me, you have believed. But since the Apostle Paul says: Now faith is the substance of things to be hoped for, the evidence of things that appear not; it is certainly clear that faith is the evidence of those things which cannot appear. The things which appear are the object, not of faith, but of knowledge.

Why then is it said to Thomas, who saw and touched: Because thou hast seen me, thou hast believed? But he saw one thing and believed another. Indeed, mortal man cannot see the divinity. So Thomas saw a man, and confessed him to be God, saying: My Lord and my God. He therefore believed through seeing, for, looking upon one who was truly man, he cried out that this was God, whom he could not see. The words which follow are cause of great joy to us: Blessed are they who have not seen, and have believed. These words are meant especially for us who cherish in our minds him, whom we do not see in the flesh. They are meant for us; but only if we carry out our faith in works. For he truly believes, who puts his faith into practice. 

Prayer to St. Thomas the Apostle

Touching the Lord's side, thou didst attain to the perfection of good things, for, as a sponge, thou drew therefrom the fount of all good and drank the draught of everlasting life, driving out ignorance from the minds of men and filling them with the doctrines of the knowledge of God.

First incredulous, then believing, thou strengthened those who were tempted, preaching to every creature how our Lord and God became flesh for us on earth and suffered death on the cross, being transfixed with nails and having His side opened with a lance, whence we draw life.

Most holy apostle, who dost rejoice in the vision of God, thou didst flood with light all the land of the Indies; when thou hadst enlightened these children of the light and the day, inspired by the Spirit, thou didst overthrow their pagan temples and didst raise their people in the love of God, to the praise and glory of the Church--O blessed intercessor for our souls.

Through a vision of the divine, thou didst become, O Thomas the Apostle, the mystic cup of the spiritual wisdom of Christ, in whom the souls of the faithful rejoice; a spiritual net, thou didst draw men from the abyss of ignorance. Hence thou became as a stream of charity from Sion, filling every creature with thy teachings of the divine. Thou didst imitate the Passion of Christ and, with thy side pierced for Him, didst put on immortality. Pray to Him for us, that He may have mercy on our souls.

    

Prayer:


O glorious Apostle Thomas! who didst lead to Christ so many unbelieving nations, hear now the prayers of the faithful, who beseech thee to lead them to that same Jesus, Who, in five days, will have shown Himself to His Church. That we may merit to appear in His divine presence, we need, before all other graces, the light which leads to Him. That light is Faith; then, pray that we may have Faith. Heretofore, our Saviour had compassion on thy weakness, and deigned to remove from thee the doubt of His having risen from the grave; pray to Him for us, that He will mercifully come to our assistance, and make Himself felt by our heart. We ask not, O holy Apostle! to see Him with the eyes of our body, but with those of our faith, for He said to thee, when He showed himself to thee: Blessed are they who have not seen, and have believed!

Of this happy number, we desire to be. We beseech thee, therefore, pray that we may obtain the Faith of the heart and will, that so, when we behold the divine Infant wrapped in swaddling-clothes and laid in a manger, we may cry out: My Lord! and my God! Pray, O holy Apostle, for the nations thou didst evangelise, but which have fallen back again into the shades of death. May the day soon come, when the Sun of Justice will once more shine upon them. Bless the efforts of those apostolic men, who have devoted their labours and their very lives to the work of the Missions; pray that the days of darkness may be shortened, and that the countries, which were watered by thy blood, may at length see that kingdom of God established amongst them, which thou didst preach to them, and for which we also are in waiting.