In his theology, St. Thomas Aquinas synthesizes various principles that characterize the various intellectual traditions which he appropriated. St. Thomas’ theology is based fundamentally on the authority of revelation, yet understood according to the philosophical principle of instrumental causality. Theology begins with the truth of Sacred Doctrine, the truth of God’s knowledge of Himself and of humans’ as being ordered to Him as to an end. Since God alone can impart His knowledge of Himself, the act of revelation by which it is given, and the act of faith, by which it is received, are fundamentally God’s actions. Yet this knowledge is imparted to humans, by humans and for humans. And since it is a principle of Thomistic thought that “whatever is received is received according to the mode of the receiver,” revelation is also a human act conditioned by the human. The truth of Faith is transmitted through Sacred Doctrine and is the human participation in divine science, i.e. the knowledge which God and the blessed share in heaven. Theology, insofar as it is distinct from Sacred Doctrine, is a human science of the divine. However, both start first with God and then proceeds according to the human. Theology in fact differs from Sacred Doctrine only to the extent that in theology the truth of faith is explicated through the more conspicuous use of rational arguments. In fact, theology, when properly done, will merely present all of, and only, the truth of Sacred Doctrine in another form. This is possible because of Thomas’ conviction that reason of itself can attain truth (yet not the same truths which are illumined by faith). The human, by employing faith and reason together, can attain the truth about divine things since both are legitimate means of attaining truth.
These principles by which St. Thomas understands the structure of theology are an application of principles learned from Aristotle and Plato and applied to the reality of Christian revelation. At the core of his theological synthesis is what is fundamentally a philosophical doctrine, i.e. the real distinction between essence and esse (“to be,” or existence). Since in all of creation a thing’s esse is limited by its essence, the only way to account for it existing at all is through unlimmited esse causing it, and this we call God. By his metaphysics of esse, Thomas combines God’s causality of creation (understood in Aristotelian terms) with creation’s participation in the divine (understood in neo-Platonic terms). The combination of these two traditions allows him to justify true rational knowledge of God through analogy. Creation is, by analogy, like God since He created it. And in receiving being from God, it imitates and emanates from Him and tends toward Him who is perfect Being by tending toward the perfection and continuation of its own being.
This last principle of emanation and return provides St. Thomas with the structure of his Summa Theologiae. The Summa is organized in three parts: the First Part deals with God and his creative activity (emanation); Second Part treats of human actions, along with their virtues, by which God is united to human beings in the communion of knowledge and love (return); finally, Christ and his Church are treated in the Third Part as the particular and historical means, necessitated by the Fall of Adam and Eve, of this return.
Besides the emanation and return that pertains to all created natures, these principles also apply to rational creatures according to knowledge and love. Since all things tend toward God as toward their end, humans too tend toward communion with God in knowledge and love. Since God is beyond what the human can naturally attain by itself, God gratuitously provides the means of finding happiness in communion with Himself. Grace, then, is that character or quality in humans by which we are related to God in knowledge and love. As being an effect of God upon the human, it is created, and the result of God’s efficient causality. As the means of intellectual and affective communion with God, or God’s presence in our souls in which we find beatitude and our final end, it is uncreated and God is our final cause. And as it produces effects in us in which we imitate God’s wisdom and compassion, God is the exemplar.
In Christ, an effect of God is united or returned to Himself in a manner that extends beyond participated existence, or rational communion. In Christ, God is united to creation and humanity in God’s own personal existence. Thus, the Second Person of the Trinity is efficient cause of the humanity of Christ, God acting in a temporal way. As being united personally to God, Christ’s humanity is the perfection of human communion and its final cause. And as the perfect human (being God’s perfect action in a human manner), Christ is humanity’s exemplar.