One of the things that annoys me when discussing the doctrine of the eternal generation of the Son, is when objectors start to bring the concept down to a creaturely level only to mock it and thereby, they think, dispose of it.
But the church fathers of the fourth century who, in response to Arianism, articulated the church's mature doctrine of the Trinity, understood that the eternal generation of the Son is non-corporeal and non-temporal. This follows from the very nature of God himself. God is not a created being composed of parts. He has no beginning or end. He is immutable, not subject to change. He is not confined to created processes that require time and space. The simplicity, aseity and impassibility of God are all relevant here. (These may be called meta-attributes because these terms are efforts to get at something more profound than a particular attribute. They tell us something about the very nature of God as God, in distinction from all created reality.) The point is, given God's impassibility and eternality, the generation of the Son must also be impassible and eternal.
Here are some quotes from Gregory Nazianzen, Athanasius, and Cyril of Jerusalem on this:
The eternal generation of the Son is “beyond the sphere of time, and above the grasp of reason” (Gregory Nazianzen, Third Theological Oration, NPNF2 7.302).
“Let every corporeal inference be banished on this subject” (Athanasius, De decretis 24, NPNF2 4.166).
“Whereas it is proper to men to beget in time, from the imperfection of their nature, God’s offspring is eternal, for His nature is ever perfect” (Athanasius, Against the Arians I.14, NPNF2 4.315).
“Authors of blasphemy, verily, are these foes of God! who, sooner than confess that the Son is the Father’s Image, conceive material and earthly ideas concerning the Father Himself, ascribing to Him severings and effluences and influences. If God be not a man, as He is not, we must not impute to Him the attributes of a man” (Athanasius, Against the Arians I.21, NPNF2 4.319).
Here I paraphrase Athanasius: The Arians ask “silly women” if they had a son before bearing one. And since it is obvious that women do not have sons before they bear them, they apply the same to the Son and conclude that the Son did not exist before his generation. But they might as well ask an architect whether they build without materials, and then conclude that God could not make the universe without materials. Or ask every man if he can be without place, and then conclude that God is confined in place. “... till they end in groveling with Manichees” (Athanasius, Against the Arians I.22-23, NPNF2 4.320).
“God is not a man; for men beget passibly, having a transitive nature, which waits for periods by reason of its weakness. But with God this cannot be; for He is not composed of parts, but being impassible and simple, He is impassibly and indivisibly Father of the Son … That none may think of the Offspring humanly, while signifying His essence, [Scripture] also calls Him Word, Wisdom, and Radiance, to teach us that the generation was impassible, and eternal, and worthy of God” (Athanasius, Against the Arians I.28, NPNF2 4.322-3).
“On hearing of a Son, understand it not merely in an improper sense, but as a Son in truth, a Son by nature, without beginning … a Son eternally begotten by an inscrutable and incomprehensible generation … God is a Spirit; He who is a Spirit has spiritually begotten, as being incorporeal, an inscrutable and incomprehensible generation … And whenever you hear of God begetting, sink not down in thought to bodily things, nor think of a corruptible generation, lest you be guilty of impiety. God is a Spirit, His generation is spiritual: for bodies beget bodies, and for the generation of bodies time needs must intervene; but time intervenes not in the generation of the Son from the Father” (Cyril of Jerusalem, Catechetical Lectures 11.4-7, NPNF2 7.64-6).
NPNF2 = Nicene and Post-Nicene Fathers, Second Series, edited by Schaff and Wace.
For more, see my paper on the subject.