When we argue with Arians, our main focus is on the deity of the Son. Arians and their modern heirs reject the Nicene Creed, which was written in response to Arianism, and which claimed that the Son was “God from God, Light from Light, Very God from Very God.”
Such was the focus of Athanasius’ anti-Arian treatises. He argued that it didn’t make any sense to talk about God without a Son. If God is eternally a Father, then He must eternally have a Son, since that’s what Fatherhood means. If God isn’t eternally Father, but became Father, then God is changeable, and the God of the Arians is just another mutable deity like Jupiter or Mars. If God is eternally light, He must eternally have a radiance; you can no more conceive of the Father without the Son than you can think of a light source without the rays of light that shine out from it.
Intriguingly, Gregory of Nyssa, a later anti-Arian writer, highlighted a different problem with Arianism. Instead of focusing only on the question of the Son’s deity, or the relation of Father and Son, Gregory said that Arianism misunderstood what it means for God to be eternal.
Arians defined God as the being that has no beginning. “Ungenerate” was virtually a synonym for God in the Arian vocabulary. As a result, they denied that the Son is fully God, since He was generated by the Father. In response, Gregory insisted that God’s eternity, His infinity over time, runs in both directions. To say that God is infinite is to say that He has no beginning, to be sure; but God’s eternity also means that He has no end. The Arians are wrong to consider “no beginning” to be the crucial mark of divine life. “No end” is equally important.
In a sense, Gregory took “no end” as more important. As the Lutheran theologian Robert Jenson has emphasized, Gregory said that if the Arians are going to choose between an eternal past and an eternal future, then they should emphasize the latter. He drew an analogy with human existence: “the past period of his life is nothing to him who has lived it, and all his interest is centered on the future and on that which can be looked forward to, that which has no end will have more value than that which has no beginning. So let our thoughts upon the divine nature be worthy and exalted ones; or else, if they are going to judge of it according to human tests, let the future be more valued by them than the past, and let them confine the being of the Deity to that, since time’s lapse sweeps away with it all existence in the past, whereas expected existence gains substance from our hope” (Against Eumonius, 1.42).
God is eternal because for Him both beginnings and ends are negated, but if we have to choose one or the other, we should choose to say that God’s eternity lies in His boundless future: “If . . . they are determined thus to divide the thought of eternity, and to make the one fall within the realm of that being, and to reckon the other with the non-realities of Deity . . . I would advise them to reverse their teaching, and to count the unending as being, overlooking the unoriginate rather, and assigning the palm to that which is future and excites hope, rather than to that which is past and stale.” Arians “define God’s being by its having no beginning.” Gregory said that the key mark of the biblical God is that He has no end: The “mark of deity endless futurity."
Gregory emphasized the future because future is a mark of life. Dead things have a past but no future. To say God is eternal, that He has no end, is to say He is infinitely, boundlessly alive, and always has been. To say He is from all eternity is to say that there is no cause beyond Him. You can’t go back behind God to answer the question “What caused God?” To say He is to all eternity is to say that nothing will bring His life to the standstill of death or frustrate His actions. God’s eternal rule, His eternal kingdom, will never die.
Gregory summarized the orthodox view of eternity this way: “The orthodox theory allows these words, I mean Ungenerate, Endless, to be indicative of God's eternity, but not of His being; so that Ungenerate means that no source or cause lies beyond Him, and Endless means that His kingdom will be brought to a standstill in no end. You are the same, the prophet says, and Your years shall not fail, showing by art that He subsists out of no cause, and by the words following, that the blessedness of His life is ceaseless and unending.”
All this seems abstract and philosophical, but it’s at the heart of the gospel. The gospel announces the resurrection of Jesus. It is the good news that death could not hold Him, that God has overcome this apparently immovable obstacle to human life and a human future. That not only gives us hope, but reveals God as the eternal God. Our God is not simply Alpha; He is also, eternally, Omega, eternally fulfilled as the future God that He always will be. Through Jesus, this eternal God has proven Himself to be eternal, the God who overcomes all boundaries and obstacles, the God who always has a future, the God who always makes a future for His people.