God was declared Trinitarian at the Council of Nicaea in 325 AD. The divine nature is shared amongst all three persons, who are coeternal. This belief was made dogma and thus is held as the universal belief of the Church. This decision instigated a further theological pursuit to clarify this definition of God as Trinity. Boethius engaged in this endeavor in his De Trinitate. First, he determines his own interpretation of Aristotelian metaphysics, next how to express God metaphysically as a substance, and then how to define each person metaphysically as a relation. In his Summa Theologica, Aquinas continues and invokes Boethius work. He further classifies God, according to his processions and relations, and then he discusses how to assert personhood of God and the plurality of persons in the divine nature. He finishes by detailing what are appropriate terms for referring to plurality of persons and unity in God. This Thesis pursues their primary thrust into defining the Trinitarian nature of God.