What is the biblical support for the Nicene Creed's statement of the Son being "eternally begotten"?
Traditionally, the doctrine of the eternal generation of the Son was supported by an appeal to the five Johannine texts in which Christ is identified as monogenes (Jn 1:14, 18; 3:16, 18; I Jn 4:9). As early as Jerome's Vulgate, this word was understood in the sense of "only begotten" (unigenitus), and the tradition was continued by the Authorized Version.
Irons admits that modern scholars often reject this understanding of monogenes, but he contends that the traditional understanding is preferable. Following a textual variant, he renders John 1:18 as follows:
No one has ever seen God, but the only begotten God, who is in the Father's bosom, has made him known
Now we must ask, in what sense is Jesus begotten? Our experience with "begottenness" is always temporal – a baby does not exist, and then it is begotten and comes into existence. The fathers at Nicaea, responding to Arianism, wanted to defend the eternity of the Son, so they said "eternally begotten" in their creed. Though perhaps Schaff's translation of the phrase is more accurate:
begotten of the Father before all worlds
Such language is reminiscent of several biblical passages, like John 17:5:
And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was.
And Colossians 1:15–17:
Who is the image of the invisible God, the firstborn of every creature [...] 17 And he is before all things, and by him all things consist.
The framers of the [Nicene] creed interpreted the Johannine monogenes in the traditional sense [...] The fathers of Nicea seem to have believed that the biblical teaching regarding the generation of the Son (as indicated by the term monogenes) was powerful evidence that he is homoousios with the Father!
While Paul's epistles and all four gospels inform us that Jesus is the Son of God, it is not until John's Gospel that we are told that Jesus was begotten from the beginning of time; the synoptic gospels appear to allow that Jesus was begotten in human time. John 1:1 tells us that the Word was with God in the beginning, John 1:14 identifies the Word with Jesus, begotten of the Father and in John 1:34, John the Baptist bears record that Jesus is the Son of God. In combination, these passages provide biblical support for the Nicene Creed's statement of the Son being "eternally begotten":
John 1:1: In the beginning was the Word, and the Word was with God, and the Word was God.
John 1:14: And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
John 1:34: And I saw, and bare record that this is the Son of God.
While the Bible may be considered by some to be ambiguous on this, the Nicene Fathers did not consider the incarnation (John 1:14, "the Word was made flesh, and dwelt among us") to be the moment when Jesus was begotten. This is apparent in the Nicene Creed of 325, where Jesus is begotten as being of one substance with the Father:
And 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;
That the Nicene Fathers and their immediate successors considered Jesus as begotten before the incarnation is made even clearer in the revision that came out of the First Council of Constantinople (381):
And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all worlds (æons) [eternally], Light of Light, very God of very God, begotten, not made, being of one substance with the Father;