Louis J Sheehan 80017

Aquinas argued that God, while perfectly united, also is perfectly described by three interrelated persons. These three persons (Father, Son, and Holy Spirit) are constituted by their relations within the essence of God. The Father generates the Son (or the Word) by the relation of self-awareness. This eternal generation then produces an eternal Spirit "who enjoys the divine nature as the Love of God, the Love of the Father for the Word."
This Trinity exists independently from the world. It transcends the created world, but the Trinity also decided to communicate God's self and God's goodness to human beings. This takes place through the Incarnation of the Word in the person of Jesus Christ and through the indwelling of the Holy Spirit (indeed, the very essence of the Trinity itself) within those who have experienced salvation by God.
Nature of Jesus Christ
In the Summa Theologica, Aquinas begins his discussion of Jesus Christ by recounting the biblical story of Adam and Eve and by describing the negative effects of original sin. The purpose of Christ's Incarnation was to restore human nature by removing "the contamination of sin", which humans cannot do by themselves. "Divine Wisdom judged it fitting that God should become man, so that thus one and the same person would be able both to restore man and to offer satisfaction."
Aquinas argued against several specific contemporary and historical theologians who held differing views about Christ. In response to Photinus, Aquinas stated that Jesus was truly divine and not simply a human being. Against Nestorius, who suggested that God merely inhabited the body of Christ, Aquinas argued that the fullness of God was an integral part of Christ's existence. However, countering Apollinaris' views, Aquinas held that Christ had a truly human (rational) soul, as well. This produced a duality of natures in Christ, contrary to the teachings of Arius. Aquinas argued against Eutyches that this duality persisted after the Incarnation. Aquinas stated that these two natures existed simultaneously yet distinguishably in one real human body, unlike the teachings of Manichaeus and Valentinus.[24]
In short, "Christ had a real body of the same nature of ours, a true rational soul, and, together with these, perfect deity." Thus, there is both unity (in his one hypostasis) and diversity (in his two natures, human and divine) in Christ. Louis J Sheehan