The exact phrase 'the eternal Son' does not occur in the Bible, but this does not mean that it is not a biblical concept. If we were to elimate all phrases that do not occur exactly in the Bible we would be greatly impoverished; indeed we would end up being unbiblical in our communication of God's truth. For example, the exact phrase 'justification by faith' does not occur in the Bible, yet it is most definitely a biblical truth clearly taught by the Bible.
The biblical basis and validation of the term 'the eternal Son' is present in both the Old and New Testament wherever Jesus, the Son, is identified as God. Once we accept the biblical testimony that Jesus is God then of necessity we also understand that Jesus, the Son, is eternal.
Consider the following verses:

Isaiah 9:6 defines the 'son' as 'the Mighty God' and 'the everlasting Father'.
Isaiah 6:3 and 5 identify God as 'the LORD Almighty' where 'LORD' is an English translation of the Hebrew for 'I AM' - the holy, unique self-identifying personal name of God [see Exodus 3:14] which proclaims God's eternal existance - without beginning, without ending. In John 12:41 John states that Isaiah in this vision saw and wrote about the glory of Jesus Christ. In other words, this glorious being of Isaiah 6, whose name is 'I AM' - the ever-present One, is Jesus Christ, the Son.
In John 8:58 Jesus deliberately took this divine name [as he did on several other occasions] and its intrinsic concept of eternality and used it of himself. The listening Jews were immediately aware of what he was doing, and straight away grabbed some stones to kill him for blasphemy [for making himself equal with God; see also John 10:30,31]. They had no doubts about what he was claiming in using this name and its associated eternality.
Jesus, in John 17:5 refers to the glory that he had with the Father before the world began.
Colossians 1:19 and 2:9 teach us that all the fullness of God is in Jesus Christ - all that God is, Jesus is.
Philippians 2:6 refers to Jesus as 'being in very nature God'. If God is eternal, then Jesus is also eternal because his nature is the exact nature of God. He is, in essence, all that God is.
In Revelation God and Jesus the Son speak of themselves with parallel designations, each of which is a reference to eternality:

    • 'the Alpha and the Omega' [God in 1:8 and 21:6; Jesus in 22:13, and also in 1:11 in some manuscripts],
    • 'the First and the Last' [Jesus, in 1:17; 2:8; 22:13; also in 1:11 in some manuscripts],
    • 'the Beginning and the End' [God in 21:6; Jesus in 22:13]

1John 5:20 teaches us that Jesus 'is the true God and eternal life'. Both of these concepts indicate that the Son is indeed eternal. Because he is the true God he by his very essence is eternal; because he is 'eternal life' he cannot be anything other than eternal.

These are not the only biblical evidence of the validity of the term 'eternal Son', but they give more than sufficient evidence of the validity of applying the term 'the eternal Son' to Jesus Christ.

© Rosemary Bardsley 2015