The commentary from the Catholic Encyclopedia is in italics and my comments in bold. The following is as close as I can find to the “conversion” of Thomas:
Landulph, his father, was Count of Aquino; Theodora, his mother, Countess of Teano. His family was related to the Emperors Henry VI and Frederick II , and to the Kings of Aragon, Castile, and France. Calo relates that a holy hermit foretold his career, saying to Theodora before his birth: “He will enter the Order of Friars Preachers, and so great will be his learning and sanctity that in his day no one will be found to equal him” (Prümmer, op. cit., 18). Beloved, there is one and one alone who has not equal among men, and that is the man, the Mashiach, Yeshua of Nazareth. At the age of five, according to the custom of the times, he was sent to receive his first training from the Benedictine monks of Monte Cassino. Diligent in study, he was thus early noted as being meditative and devoted to prayer, and his preceptor was surprised at hearing the child ask frequently: “What is God?” Grace to accept the one true gospel answers the question “who is God?” The God and Father of our Lord Yeshua ha Mashiach. Our fellowship is with the Father and His Son. God reveals Himself by two persons, the Father and the Son through the Spirit of the Father. For it was the Father who gave the Spirit unto Yeshua, and Yeshua who gave gifts of grace unto the chosen ones.
About the year 1236 he was sent to the University of Naples. Calo says that the change was made at the instance of the Abbot of Monte Cassino, who wrote to Thomas’s father that a boy of such talents should not be left in obscurity (Prümmcr, op. cit., 20). At Naples his preceptors were Pietro Martini and Petrus Hibernus. The chronicler says that he soon surpassed Martini at grammar, and he was then given over to Peter of Ireland, who trained him in logic and the natural sciences. The customs of the times divided the liberal arts into two courses: the Trivium, embracing grammar, logic, and rhetoric; the Quadrivium, comprising music, mathematics, geometry, and astronomy . . . . Thomas could repeat the lessons with more depth and lucidity than his masters displayed. The youth’s heart had remained pure amidst the corruption with which he was surrounded, and he resolved to embrace the religious life. All that is described here is a young man who was gifted in the flesh. From what I examine of Thomas’ life, I am not able to make a judgment of whether Thomas was converted or not. What is written of his life is a declaration he would be great before his birth, he came from nobility, and he had extraordinary talents that were recognized from an early age. I see no one true gospel. Thus by what is written there is no gospel. And the abundance of Thomas doctrine and his pursuit of proving God intellectually, barren of the declaration of the one true gospel, barren of the supremacy of the one true gospel, shows to me by further writings that grace was never there or he resisted and departed from grace. God makes no mistakes. Beloved, judge all things by the gospel. It does not matter how many affirm that a man is of God. For many follow after opinions rather than the thoughts of God.