There is little, if anything, in the Nicene formulae which is not present in embryonic or directional form in the works of Irenaeus. The cardinal place he holds in the way in which orthodox theology opens out to the full expression and significance of the Nicene faith has been underestimated generally in the lack of any systematic consideration of the relation between the influence he left and the achievement of the Nicene fathers, and particularly in the failure to accord his methodology in theology the foundational, directional and formative place it occupies. A comparison of what his work set out and clarified with the achievements of the Nicene fathers, for example, would suggest that the general thought established by Irenaeus is the cardis, on which they were able to move– the former was the hinge of the latters' door, both leading to that theological panorama set out by the Councils (and all the achievement of what lay behind them) of ad325, 381 and 451. Such themes are:
The internal and eternal relations of Father, Son and Holy Spirit, particularly the ground laid by Irenaeus of the δμοουσιος, the same substance, of the Son and the Father; the insistence that the Word is semper co-existens, ever co-exists with the Father; and hand in hand with this the oft repeated clear implication that the Spirit is likewise, his co-eternity being explicitly set out in Adversus Haereses IV:20:1,3, for example.
The external acts of God, showing that there is no disjunction between the Being and the activity of God by emphasizing clearly the mode of the Self-humbling of the eternal Son in the διχουομια, the ordering or dispensation of the Incarnation, thereby implying that what God is eternally in Himself, He is towards us in Jesus Christ.
The refusal to distinguish the Father from the Creator. There is one God who in the being of His Fatherhood is the Creator of all through His Son who is the Word, and the Holy Spirit.
The Royal Exchange and the Deification of Man by grace (that is, humanity's participation in the Divine life without ceasing to be human). The Son becomes what we are in order that we might become what he is. The parallel here with Athanasius's De Incarnations LIV:3 is remarkable. There is also the emphasis here that Christ is God come as man, assuming the totality of what makes human existence. Therefore faith and godliness necessarily go hand in hand – the ground of Athanasius's dictum that we are to live a life in correspondence to God.
The growth of the child Jesus in wisdom and favour with God and man – the πϱοεχοπτε, the cutting his way forward. Here is the foundation of Athanasius's comment on Christ ministering the things of God to man and the things of man to God. It is also the hinterland of the observations by Basil and Gregory Nazianzen, in their respective ways, on the opening of the way for humanity to rise to knowledge of God. The emphasis on faith seeking understanding – that we believe in order to understand, rather than understand so that we may believe, set out definitively by Anselm of Canterbury so much later – is also found here as fundamental to Irenaeus's theology, in harness with the idea of God being beyond comparison, more than the comprehension of human thought, and elusive to description by human words.
30The Holy Spirit, our knowledge of God and our godliness. Irenaeus is clear that the Spirit is not a creature. The Spirit is related inseparably to the Son, and, although the δμοουδl;ιιυ of the Spirit is not drawn out, it is implied. The coeternity of the Spirit with the Word and the Father is set out for example, in Adversus Haereses IV:20:1,3, as noted above. It is the Spirit who brings us into a community of union with God through Christ; we are raised by the Spirit to knowledge and communion with God and this can be an act of God alone. This communion is a sharing of the knowledge which God has of himself in his eternal Triadic relation.