The early church held to the eternal generation of the Son of God, which can be defined as “the unique property [characteristic, attribute] of the Son in relation to the Father. Since God is eternal, the relation between the Father and the Son is eternal” (Robert Letham, The Holy Trinity, 499). Augustine provided a helpful analogy: it’s not like “water flowing out from a hole in the ground or in the rock, but like light flowing from light” (De trin. IV.27, 172).
Keith Johnson summarizes the teaching in a bit more technical detail: “the Father eternally, necessarily, and incomprehensibly communicates the divine essence to the Son without division or change so that the Son shares an equality of nature with the Father yet is also distinct from the Father” (“Augustine, Eternal Generation, and Evangelical Trinitarianism,” TrinJ 32 : 141-163).
This understanding is reflected in the original wording of the Nicene Creed (325 AD):
[We believe] in one Lord Jesus Christ, the Son of God, begotten of the Father, the only-begotten; that is, of the essence of the Father, God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father.
If you want a very careful summary and introduction to Augustine’s De trinitate (On the Trinity), see Johnson’s Rethinking the Trinity and Religious Pluralism: An Augustinian Assessment (IVP, 2011). For those, like me, who aren’t actively interesting in the issue of pluralism as it relates to the Trinity, I would recommend simply reading his summary exposition of Augustine’s work.
The doctrine has fallen on hard times, however, and some of the evangelical theologians I admire the most don’t think that the idea is biblical.
A full exegetical defense is obviously beyond the scope of a blog post, but I thought it might be helpful to excerpt a couple of sections from those who do think the idea is biblically defensible and theologically indispensable. These selections provide some helpful angles on this ancient doctrine.
First, in his new systematic theology—written with no footnotes except for biblical references!—Gerald Bray writes in God Is Love: A Biblical and Systematic Theology (Wheaton, IL: Crossway, 2012):
The framework within which we must understand the true meaning of “generation” with respect to the second person of the Trinity is established by the discipline of law, not by biology or grammar. As a legal principle, “generation” is fully consistent with the covenant principle and the structure of Old Testament revelation that provides the basic framework for understanding God’s saving work. The relationship of the first to the second person of the Trinity is based on the concept of inheritance. The Son has been appointed the heir of all things, and it is in this sense that he is described as the firstborn of all creation [Col. 1:15-16; Heb. 1:2; see also Gal. 4:7]. By the law of primogeniture, the firstborn is the one who receives the inheritance, as the story of Esau and Jacob reminds us [Gen. 27:18-35; Jacob (the younger son) stole his brother's inheritance by impersonating him in the presence of his blind father Isaac].
In human affairs the language of inheritance is normally attached to the process of birth and human reproduction, but not always—it is quite possible to leave a legacy to someone totally unrelated to the donor, or to adopt others with the intention of giving them an inheritance, as God has done with us [Gal. 3:29; Eph. 3:6]. The concept is flexible and can be applied to different situations by choice, not merely by necessity, which makes it so valuable for understanding both our relationship to God and the relationship of the Son to the Father.
The use of Father-Son language serves the additional purpose of emphasizing the underlying unity of being that binds them together and makes it possible for us to say that the Son is the Father’s natural heir, whereas we are heirs only by the grace of adoption. “Like father, like son” is a well-known popular expression that conveys the essence of this and reminds us that, just as a human child shares the nature of his parents, so the Son of God must share the nature of his Father if he is to be a genuine Son. The shared being and nature are inherent in the terminology used to describe them. It is true that angels, and perhaps other heavenly beings, are occasionally called “sons of God” in the Old Testament, but this refers to their spiritual nature, which is like that of God himself, and is not connected to any inheritance that might be reserved for them [Gen. 6:2-4; Job 1:6; 2:1]. In that sense, the Son is the only begotten, as the prologue to John’s Gospel specifies and as the creeds of the early church repeat [John 1:14].
D. A. Carson, writing in his Gospel according to John Pillar commentary (Grand Rapids: Eerdmans, 1990), looks at a very interesting verse: “For as the Father has life in himself, so he has granted the Son also to have life in himself” (John 5:26). Carson writes:
The logical For (gar) is important: this verse explains how it is that the Son can exercise divine judgment and generate resurrection life by his powerful word. It is because, like God, he has life-in-himself. God is self-existent; he is always ‘the living God’. Mere human beings are derived creatures; our life comes from God, and he can remove it as easily as he gave it. But to the Son, and to the Son alone, God has imparted life-in-himself.
This cannot mean that the Son gained this prerogative only after the incarnation. The Prologue has already asserted of the pre-incarnate Word, ‘In him was life’ (1:4). The impartation of life-in-himself to the Son must be an act belonging to eternity, of a piece with the eternal Father/Son relationship, which is itself of a piece with the relationship between the Word and God, a relationship that existed ‘in the beginning’ (1:1). That is why the Son himself can be proclaimed as ‘the eternal life, which was with the Father and has appeared to us’ (1 Jn. 1:2).
Many systematicians have tied this teaching to what they call ‘the eternal generation of the Son’. This is unobjectionable, though ‘the eternal generation of the Son’ should probably not be connected with the term monogenēs (sometimes translated ‘only begotten': cf. notes on 1:18). In the immediate context, it is this eternal impartation of life-in-himself to the Son that grounds his authority and power to call the dead to life by his powerful word.