Thomas Aquinas applied himself to Natural Theology which is to seek knowledge of God and of things through reason. It was Aquinas who defined in detail what could be taken as true and what should be regarded as false in the many areas of theology. In the numerous volumes of The Summa Theologica Aquinas lays out his arguments for the correct understanding of God, the Trinity, angels, human souls, the human condition and creation.
Aquinas is most quoted on his five ways to prove the existence of God, although this is a very small section of Summa. Aquinas’ tour de force was the recognition that God created things from nothing: in Latin creatio ex nihilo. This became the Doctrine of Creation of the Catholic church.
Aquinas debated with Siger of Brabant who was a Latin Averroist disagreeing with him on many points. Brabant proposed that church theology is true and philosophy is true even though they contradict each other – you just keep them separate. Aquinas proclaimed one truth although this one truth could be approached by the two paths. Aquinas stood firm and would not be moved and would not accept contradiction or the double truth.
Aquinas tirelessly fought against heretics. There is an anecdote of Aquinas having been invited to a dinner with King Louis IX of France and his courtiers. He reluctantly went, however, he found the flirtatious courtiers boring so he remained in his own thoughts, not listening to the trivial chit chat. Suddenly he banged his fist on the table and exclaimed “And that will settle the Manichees.” The king quickly summoned a scribe to write down on a tablet the inspiration that Aquinas had just had to prove a point before he forgot it.