In comparing the medieval Christian philosophical, human anthropology to the dominant classical philosophers Aristotle and Plato, one of the most important fundamental distinctions between the two schools of thought is their notion of existence or essence. For St. Thomas Aquinas and his contemporaries, divine revelation led him to posit that the universe and all that is, was created by God out of nothing, creatio ex nihilo. The Christian understanding of this doctrine leads to an important distinction from the pagan philosophers: unlike Aristotle and Plato, St. Thomas and his contemporaries find there to be two acts of God's “To-Be” involved in the creation from nothing. The Christians posit that God is the sole necessary being: that all other being is contingent. It follows then that despite man's being immortal, he is existentially dependent upon God. As a creational being, man is a complete, ontological unity who needs God. As St. Thomas writes, “[I]n God, there is no potentiality” so that “in [God] essence does not differ from existence” (Summa Theologica II-II, Q. 3 a. 4). God's very essence is existence itself. Thus, man, whose essence is not existence, must be utterly contingent upon God. Man participates in God's being, and whatever is man is somehow correlated to God who is the only Being that owns his own To-Be.
For man, then, there must also be two acts of To-Be: man's existence, his perfections, and everything else that is encompassed by the term “man” is real, Good, and participating in God's own essence, which is existence itself. Man is an embodied soul: the pagan Aristotelian materiality is raised up to the intelligibiltiy of St. Thomas' embodiment of the soul. Man's unique essence is strengthened by a distinction between essence and existence: his essence become actualized and given greater fulfillment as an essence because he recognizes that he can be more than himself. Man is bound up in God, because only God owns his own To-Be, making Man a participated being. What looked like materiality cannot affect the soul or essence of man, and so God's second act of To-Be, existence itself, is made manifest: causally rooted in God's own self.
The distinction between existence and essence remains for man, however, because since man is bound up in God's own essence through his participation in God's essential being, God's existence, man cannot bring together, in his own being, the two into one. Otherwise, man himself would be dissolved into God or become some sort of demigod who possesses both existence and essence. Of course, this would be impossible, since all being and essence is already God's—God clears the field, as it were, in our creation, making it literally unattainable for one of the beings contingent upon his essence for its existence to then act to combine its essence and existence in the same manner as God's own being. The distinction is a result of God's choosing to create out of nothing, rather than, as the pagan philosophers Aristotle and Plato suppose, the universe being eternal itself. The distinction also contradicts the notion that creation occurred as a sort of emanation of God's being: where creation was not a volitional decision of God, but rather a type of necessary overflowing of God's being into a creation separate from, but constituted from, himself.
According to St. Thomas and his contemporaries, God chose to create out of nothing as an act of pure volition. There was no requirement for God to create; indeed, God is complete and utterly happy in himself without need for recourse to anything or anyone else. Nor is God's being so uncontainable that parts of God's being overflow the bounds of his existence to cause creation “by accident.” As the Christian philosophers of the medieval period argue, God's very nature—one, holy, true, beautiful, and simple—means that God only acts when he wants to and how he wants to. In the creation of the universe, God chose to create out of nothing because he could and because he wanted creatures who could participate in his essence while not sharing his existence. At the same time, he wanted creatures who were capable of more than just existence in itself. Mankind, whom God created ex nihilo with participation in his essence, can also aspire to be more than he is at present. God's gift to man of that very participation makes it possible for man to hope, by God's grace to be more fully realized. Man share's God's free will, and as a result, he is eternal but existentially dependent upon God. Man has the potential for change and growth, leading to his ultimate reunion with God without absorption: the same paradox that marks the Thomist notion that God's creatio ex nihilo leads to man's participation in God's essence while being distinct in his existence.
Man's ability to share in God's essence without losing his own existence—his participation in the two acts of To-Be by the creator God—means that man is aeviternal: once man is created, because of his sharing in God's essence, he cannot perish. Because God's essence cannot stop-being, since God is himself Being, man-who-participates in God's essence must needs be everlasting from the point of his creation ex nihilo by God. This is not to say that because man shares God's essence that man becomes eternal, meaning extant at all times and even outside of time like God is. Rather, man's aeviternity, a result of God's gift to man of participation in God's essence, means that man has an everlasting existence of one sort or another. This is radically opposed to the conception of the pagan philosophers. For Plato, reunion with the forms was not to be seen as a conscious existence, but rather as a return to the source as a glass of water is poured into the sea: the soul would not cease to exist, but it would cease to have individual identity and be freed from the limitations it experienced while embodied (or, in the example here, contained). The aeviternity of man that is evident from the Thomistic participation in God's two acts of To-Be also contradicts Aristotle's hylomoprhic understanding of the human person which ceases to be when the matter of the body is dead. While the Christian philosophers were able to grasp the implications of aeviternity from their advantage of divine revelation (through the sacred Scriptures and the magisterial tradition of the apostolic faith), it would not be necessary to hold to divine revelation to come to an understanding of man's aeviternity. Indeed, if one grasps that God is both essence and existence and that God creates ex nihilo, then aeviternity must, of necessity, follow: those things which are created out of nothingness cannot have being unless they participate in God's being, and to participate in God's essence is to be eternal, forward from the point of creation.
Theological revelation, as has been stated, gave an advantage to St. Thomas and his contemporaries in understanding the aeviternity of man and in grasping the participation that man enjoys in God's being. This revelation is God's disclosing of himself and his essence to those he has created to participate in that same essence. Because God is transcendent as the one, the holy, the true, and the good, he must show himself to his creatures so that they can have understanding of his being as such. Of course, as St. Paul argues in sacred Scripture, the general revelation of creation itself offers a glimpse into God's reality, but even then, as the name suggests, the uncovering of God through the inanimate creation to his aeviternal creation of rational men still involves God revealing himself in the creation of the environment in which they are attempting to find him.
The theological revelation of God through his general revelation can be related to human reason and the efforts of men like Plato and Aristotle through the notion that even in their pre-Christian worldview, the only things about which they had to reason were their own lives, experiences, and surroundings. Since all these were created by God, it could be argued that even though the pagan philosophers were only using their reason, they were nonetheless influenced by God's revelation of himself through that very creation: after all, it was only God's creation—including their own essence and existence—which they could reason about in the first place. Moreover, the reason which they were using to ascertain truths about the universe is a product of God's gift of his own essence to them as persons. The person par excellence, as St. Thomas argues, is God in his Trinity. But even in the aeviternal men which seek after him—either through revelation or reason—there is personhood, which yields to the seekers of truth, both pagan and Christian, those aspects of reality which they were able to find. This was the essence—that sharing in God's own being—which they, in their individual existences, were able to claim as persons, thus leading them to partake of both reason and theological revelation about their own ontology as contingent beings depending on Being qua Being.
This grasping of their own contingency, even in their personhood, leads St. Thomas and his contemporaries to understand just how far their existence, even in its ex nihilo creation, is from the nothingness from which they find themselves to be drawn. For Plato and Aristotle, relying solely on their reason to grope for the truth of their being, such an understanding is not possible. Positioned as they are with the notion that there is only one To-Be that leads to their actuality—that their essence and existence are the same—their conception of the source of their own being founders from Plato's realm of the forms to Aristotle's hylomorphism. And though both philosophers come closer to the truth than many, many other thinkers who relied solely on their own human reason and the natural, general revelation available in creation (so much so that St. Thomas is able to rely heavily upon Aristotle, and St. Augustine upon Plato), they still lose the fundamental reality despite their best effort. As St. Thomas argues, nothingness is infinitely less than somethingness. In other words, existence is infinity greater than non-existence. And this existence can only be understood because of its drawing upon God's essence as the pure, necessary Being, in whose essence men partake through participation. Considering, then, the distance between the infinite superiority of man's existence over against his non-existence, there must also be—and St. Thomas argues that there is—an equally infinite distance between man's own existence as such and God's essence as Being.
As St. Thomas grasps this reality, he must also come to face the filial fear of the realization that his essence as an actuality is only contingent: that it is God himself who, in effect, holds him and all creation in being. Upon coming to this truth, though, it is clear that St. Thomas does not shrink from the God whose essence he now understands he shares; instead, St. Thomas—with the whole of the Christian tradition—is drawn toward this self-revelatory, self-existent, self-essential God with a sort of filial fear. Not fear out of a sense of doom or terror, but, rather, out of a sense of love. The realization that one's being is contingent upon God's essence, and that one's being is but a share and participation in God's very being, yields a radical reassessment of the whole of creation. No more is man limited to Plato's or Aristotle's dismal, eternal, and infinitely lonely materialist medium for simple existence. Now, in light of God's intimate union with his creatures even so far as to hold their bodies, souls, and world in place with the substance of who he is, St. Thomas—again, with all Christendom—cannot but help to bow down in adoration to that Most High God who has made him, in one respect, his brother.