The special qualification of the second person in the Trinity is filiation. In Scripture he bears several names that denote his relation to the Father, such as word, wisdom, logos, son, the firstborn, only-begotten and only son, the image of God, image (εἱκων), substance (ὑποστασις), stamp (χαρακτηρ) [cf. Heb. 1:3]. The doctrine of “eternal generation” (αἰωνος γεννησις), so called for the first time by Origen, was based on these names and a few texts cited above. In using these terms we are of course speaking in a human and hence an imperfect language, a fact that makes us cautious. Yet we have the right to speak this language. For just as the Bible speaks analogically of God’s ear, eye, and mouth, so human generation is an analogy and image of the divine deed by which the Father gives the Son “to have life in himself.” But when we resort to this imagery, we must be careful to remove all associations with imperfection and sensuality from it. The generation of a human being is imperfect and flawed. A husband needs a wife to bring forth a son. No man can ever fully impart his image, his whole nature, to a child or even to many children. A man becomes a father only in the course of time and then stops being a father, and a child soon becomes wholly independent from and self-reliant vis-à-vis his or her father. But it is not so with God. Generation occurs also in the divine being. God’s fecundity is a beautiful theme, one that frequently recurs in the church fathers. God is no abstract, fixed, monadic, solitary substance, but a plenitude of life. It is his nature (οὐσια) to be generative (γεννητικη) and fruitful (καρπογονος). It is capable of expansion, unfolding, and communication. Those who deny this fecund productivity fail to take seriously the fact that God is an infinite fullness of blessed life. All such people have left is an abstract deistic concept of God, or to compensate for this sterility, in pantheistic fashion they include the life of the world in the divine being. Apart from the Trinity even the act of creation becomes inconceivable. For if God cannot communicate himself, he is a darkened light, a dry spring, unable to exert himself outward to communicate himself to creatures.
Still, that generation is to be conceived in divine terms. In the first place, it is spiritual The Arians, in opposing the idea of divine generation, objected that all generation necessarily brings along with it separation (τομη) and division (διαιρεσις), passion (παθος) and emanation (ἀπορροια). And that would be correct if it were physical, sensual, and creaturely. But it is spiritual, divine, and therefore simple, without division (ἀρρευστως) or separation (ἀδιαιρετως). It occurs without flux and division. While giving rise to distinction and distribution in the divine being, it does not create divergence and division. Athanasius writes: “Inasmuch as God is simple, the Father of the Son is indivisible and without passion, for although in the case of humans we speak of outflow and inflow, we cannot predicate these things of anything that is incorporeal.”120 The most striking analogy of divine generation is thought and speech, and Scripture itself suggests this when it calls the Son “Logos” [Speech, Word, Reason]. Just as the human mind objectivizes itself in speech, so God expresses his entire being in the Logos [Christ]. But here, too, we must note the difference. Humans need many words to express their ideas. These words are sounds and therefore material, sense-related. They have no existence by themselves. But when God speaks, he totally expresses himself in the one person of the Logos, whom he also “granted to have life in himself” (John 5:26 NIV).
In the second place, therefore, divine generation implies that the Father begets the Son out of the being of the Father, “God of God, Light of Light, very God of very God; begotten, not made, being of one substance with the Father,” as the Nicene symbol has it. The Arians, by contrast, contended that the Son had been brought forth by the will of the Father out of nothing. This, however, is not generation but creation, as John of Damascus points out. Creation is “the bringing into being, from the outside and not from the substance of the Creator, of something created and made entirely dissimilar [in substance],” while “begetting” means “producing of the substance of the begetter an offspring similar in substance to the begetter.” The Son is not a creature but he is “God over all, forever praised!” (Rom. 9:5 NIV). Accordingly, he was not brought forth by the will of the Father out of nothing and in time. Rather, he is generated out of the being of the Father in eternity. Hence, instead of viewing “generation” as an actual work, a performance (ἐνεργεια), of the Father, we should ascribe to the Father “a generative nature” (φυσις γεννητικη). This is not to say, of course, that the generation is an unconscious and unwilled emanation, occurring apart from the will and power of the Father. It is not an act of an antecedent decreeing will, like creation, but one that is so divinely natural to the Father that his concomitant will takes perfect delight in it. It is a manifestation of what is truly expressive of his nature and essence, and therefore also of his knowledge, will, and power, in fact of all his virtues.
In the third place, therefore, the church confesses its belief in the eternal character of this generation. The Arians said that there was a time when the Son did not exist (ἠν ποτε ὁτε οὐκ ἠν). They appealed especially to the words “he brought me forth,” or “created me” in Proverbs 8:22, and pointed out the antinomy between the terms “eternal” (αἰωνος) and “begetting” (γεννησις). But if the “Father” and the “Son” bear their names in a metaphysical sense, as Scripture incontrovertibly teaches, it follows that the generation in question has to be eternal as well. For if the Son is not eternal, then of course God is not the eternal Father either. In that case he was God before he was Father, and only later—in time—became Father. Hence, rejection of the eternal generation of the Son involves not only a failure to do justice to the deity of the Son, but also to that of the Father. It makes him changeable, robs him of his divine nature, deprives him of the eternity of his fatherhood, and leaves unexplained how God can truly and properly be called “Father” in time if the basis for calling him “Father” is not eternally present in his nature. We must, accordingly, conceive that generation as being eternal in the true sense of the word. It is not something that was completed and finished at some point in eternity, but an eternal unchanging act of God, at once always complete and eternally ongoing. Just as it is natural for the sun to shine and for a spring to pour out water, so it is natural for the Father to generate the Son. The Father is not and never was ungenerative; he begets everlastingly. “The Father did not by a single act beget the Son and then release him from his ‘genesis,’ but generates him perpetually.” For God to beget is to speak, and his speaking is eternal. God’s offspring is eternal.
—Herman Bavinck, Reformed Dogmatics: God and Creation, transl. John Bolt, and John Vriend (Grand Rapids, MI: Baker Academic, 2004), 2.308–310.