Athanasius insists that the Father must have an eternal Son because the Father’s essence could never have been imperfect:

“if He is called the eternal offspring of the Father, He is rightly so called. For never was the essence of the Father imperfect ( ateles ), that what is proper to it should be added afterwards.”  By contrast, human beings, as creatures in time, beget over time, one generation following another, to complete the imperfection of their nature ( ateles tes phuseos ).

Having a Son thus perfects, completes, brings to its fulfillment the Father’s essence.  The Father does have a telos , and that telos is reached in begetting a Son.  Since it would be unworthy of God to suggest that He ever fell short of that fulfillment, the Son must be eternal.

Now, three things: First, this implies that the Father is not complete “in Himself” (contrary to what one finds in Augustine).  Second, it is necessary for the Father to be completed in something else.  Of course, since the Son is eternal, the Father is always already reached His complete end, but the Father’s being is such that it is not completed without the Son.  He is Light, and Light, to be complete, requires radiance.  The Father is origin, but, Athanasius implies, cannot be origin without the eternal supplement of the Son.  Third, this perhaps implies a kind of eternally realized eschaton in the life of the Trinity.  The Father has always already reached the consummation; He is always already the God of the future, the God whose future is wholly realized in the Son, the God who’s always waiting to greet us down the road.