A straightforward way to understand and avoid the unnecessary Western theological complications (Augustinian and Thomistic) of a two-tiered nature/supernature split, or nature pitted against grace, is to follow Irenaeus’ argument against the Gnostics. Irenaeus’ description of soul, body and Spirit, clarifies that there is no “human nature” apart from God (or a Gnostic dualism). “Now the soul and the Spirit are certainly a part of the man, but certainly not the man; for the perfect man consists in the commingling and the union of the soul receiving the Spirit of the Father, and the admixture of that fleshly nature which was molded after the image of God.” Where the Gnostics taught a radical dualism between God and matter, and God and the world, positing a mediating deity or Demiurge between God and the world, Irenaeus, combines the Genesis story and its completion in Christ, to picture the full participation of God in man and man in God as the “natural” end (anything less is unnatural). Creation and theosis are not separate events anymore than creation and incarnation. This is who God always is; this is who man always is, and this is the point of creation.
Irenaeus describes the necessity of the Spirit of God, not as a force apart from man but as molding and blending the handiwork of God: “But when the Spirit here blended with the soul is united to God’s handiwork, the man is rendered spiritual and perfect because of the outpouring of the Spirit, and this is he who was made in the image and likeness of God.” That is, the Genesis account is only completed through the active participation of God in the man as Spirit. But this is not simply God’s Spirt but it is the constituting element of the man.
While all three elements, body, soul and Spirit, constitute the image of God in which man was created, Irenaeus use of Spirit (sometimes seeming to refer to God and man simultaneously) portrays the perfection of full co-participation between the divine and human while also allowing for a diminishment of participation: “One of these does indeed preserve and fashion (the man) – – this is the Spirit; while as to another it is united and formed–that is the flesh; then comes that which is between these two–that is the soul, which sometimes indeed, when it follows the Spirit, is raised by it, but sometimes it sympathizes with the flesh, and falls into carnal lusts.” The Spirit “preserves and fashions” the man, so that there is no human apart from Spirit. The Spirit is not something added to man, and yet there is the possibility, in following lusts, that the role of the Spirit is diminished.
As Irenaeus describes it, no part of the flesh, soul, Spirit trichotomy can be left out of the mix that makes up the complete human:
But if the Spirit is lacking from the soul, such a one, remaining indeed animated and fleshly, will be imperfect, having the image, certainly, in the handiwork (the flesh), but not receiving the likeness through the Spirit. Likewise this one is imperfect, in the same manner again, if someone takes away the image (soul) and rejects the handiwork – one can no longer contemplate a human, but either some part of the human, as we have said, or something other than a human. For neither is the handiwork of flesh itself, by itself, a perfect human, but the body of a human and a part of the human; nor is the soul itself, by itself, a human, but the soul and a part, of the human; nor is the Spirit a human, for it is called Spirit and not human. But the commingling and union of all of these constitutes the perfect human. 
John Behr notes, with Irenaeus it is the Spirit that renders human beings both living and, due to the combination with the flesh, human. It is “the Spirit that absorbs the weakness of the flesh and manifests living human beings: living, because of the Spirit, ‘their Spirit’; and human, because of the flesh.” His point in emphasizing that this is “their Spirit” is to point out “it is always the human who lives, who personalizes the life given to them by God.” The virtues and life developed in the flesh through the Spirit are not an overriding of human personality and freedom, but their completion. On the other hand, “if we take away the substance of the handiwork (flesh) we are left with ‘only the Spirit itself, which Irenaeus describes as ‘the Spirit of the human or the Spirit of God’. The Spirit itself is not the human, nor even a part, of the human, but is given to the human in such a manner that it can be legitimately described as their Spirit.” This is not simply the animating breath but the vivifying Spirit of God, which is their Spirit.
The image (but not the likeness) may be present, where the soul and flesh are joined apart from the Spirit, but clearly the Spirit is needed to complete the creation account – in the “image and likeness.” Though already part of the original creation account, Irenaeus envisages (through his quotation of I Thess. 5:23) the completeness (the fullness of image and likeness) as occurring in the eschaton. But this completion is a process initiated from the moment of creation.
Part of the problem for later interpreters of Irenaeus, shaped by a Catholic confessional understanding (making a distinction between nature and grace), is that Irenaeus distinguishes between image and likeness, but not so as to posit the possibility of an image (“natural”) apart from the likeness (which he ties to the Spirit), as this would amount to an unformed or unmolded man. No part of man can be completely separated from God and still be human. An unspiritual human or an ungraced man is not of the human species at all, such that there is not the possibility of a man apart from the grace of God.
As Dai Sal Kim has put it, “The fact that man was created by God was a pure grace, to begin with! Then how can we say that the addition of the spirit is the supernature added by the grace of God?” As Vladimir Lossky portrays it, the Eastern tradition will preserve the understanding of the image of God as preserving the full integration of nature and grace. He writes, “The idea of supererogatory grace which is added to nature in order to order it towards God is foreign to the tradition of the Eastern Church. As the image of God, the ordering of the human person was towards its Archetype.”
Allan Galloway points to Augustine as the founder of the dualism between nature and grace, developed and aggravated in the Middle Ages. He compares Irenaeus to Augustine, indicating that while Augustine acknowledged the goodness of nature it was an insignificant goodness in comparison to the benefits of Christ, while in Irenaeus there is only one order of goodness. “All goodness, whether it belongs to this world or to the final consummation, is a manifestation of the grace of God.” In Irenaeus there is no trace of the dualism of the Middle Ages as there is a complete and ultimate unity of nature and grace.
For Irenaeus, humans are not two-storied creatures, first possessing an integrated human nature, to which relationship to God is added. By definition, the image and likeness in which humans are created is a participation in the divine. Grace is not added to an ungraced world or to a God free man, as what it means to be made soul, body, and Spirit created in the image and likeness of God is the participation of God in man and man in God.
 Against Heresies (AH) Book 5, Chapter 6, paragraph 1.
 AH 5.6.1
 AH, 5.9.1.
 AH 5.6.1.
 John Behr, Godly Lives: Asceticism and Anthropology, with special reference to Sexuality, in the Writings of St. Irenaeus of Lyons and St. Clement of Alexandria (Pembroke College, Oxford, 1995), Abstract.
 Behr, 118-119.
 Dai Sil Kim, The Doctrine of Man in Irenaeus of Lyons, (Boston University: Doctoral Dissertation, 1969) 96-97.
 Vladimir Lossky, The Mystical Theology of the Eastern Church, (London: James Clarke & Co. Ltd., 1957) 131. Quoted in Kim, 97.
 Allan D. Galloway, The Cosmic Christ, (New York: Harper & Brothers, Publishers, 1951) 127. Quoted in Kim, 97.