Originating from the revelation of Christ, trinity by nature is part of Christianity and therefore a mystery beyond the level of reason. But faith is to search for understanding; therefore, numerous thinkers have spent their whole lives exploring this faith ever since the beginning of the church. This is also how Thomas Aquinas, the great medieval theological philosopher, set to work on his masterpiece, Summa Theologica. And his thought made tremendous influence on the theological development of trinity. This article is first and foremost about Aquinas' idea of trinity. The first part is to introduce the biblical foundation of trinity, along with the controversy produced in the early church during the theological construction of trinity. The second part roughly describes how Aquinas' theology of trinity was influenced by St. Augustine as well as the philosophical context of trinity in the first part of Summa Theologica. The third part of this paper discusses Aquinas' thought before the treatise on trinity, that is, "The One God." In this treatise, Aquinas exploits the idea of existence from Aristotle's metaphysics so as to illustrate the oneness and dynamics of the nature of God as the foundation of the interpretation of trinity. The fourth part points out how Aquinas explains the rationality of the faith of trinity through the person of metaphysical relation. And, finally, it's suggested that Aquinas' treatise on the most holy trinity is related to all creatures and eliminates the possible alienation between the eternal life within and the external creatures without.
