Why You Should Affirm Eternal Generation

 Eternal generation is a doctrine that relates to more classical trinitarianism, it is thus defined by Charles Hodge:

The eternal generation of the Son is commonly defined to be an eternal personal act of the Father, wherein by necessity of nature, not by choice of will, he generates the person (not the essence) of the Son, by communicating to him the whole indivisible substance of the Godhead, without division, alienation, or change, so that the Son is the express image of His Father’s person, and eternally continues, not from the Father, but in the Father, and the Father in the Son

This doctrine was also taught by many leading dispensationalist teachers such as John Walvoord and Lewis Sperry Chafer:

 The very nature of procession points to its eternity. Procession like the eternal generation of Christ is not a matter of creation, commencement of existence, or analogous in any way with physical relationships common in the human realm. It proceeds rather from the very nature of the Godhead, being necessary to its existence. Without the Holy Spirit, the Godhead would not be what it is. The procession of the Holy Spirit cannot be compared to the incarnation, as the incarnation was not essential to deity, though it is essential to its manifestation, especially the attributes of love and righteousness as they combine in grace. (John Walvoord, The Person of the Holy Spirit)

John Walvoord

The theological term eternal generation implies that without beginning or ending, the Second Person is the manifestation of the Godhead. It is thus that the “only begotten Son” hath declared God to man (John 1:18). The Son said, “I have manifested thy name unto the men which thou gavest me out of the world” (cosmos—John 17:6; cf. 1 John 1:2; 4:9). He was Only Begotten in the uniqueness of His begetting.  (Systematic Theology, Lewis Sperry Chafer)

Is it Scriptural? 

There are multiple scriptural references which suggest this doctrine:

John 3:16: For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.

John 5:26, “For just as the Father has life in Himself, even so He gave to the Son also to have life in Himself.”

John 6:57-58, “57 As the living Father hath sent me, and I live by the Father: so he that eateth me, even he shall live by me. 58 This is that bread which came down from heaven: not as your fathers did eat manna, and are dead: he that eateth of this bread shall live for ever."

Another text to explicitly affirm this doctrine is Hebrews 1:3 which says:

who being the brightness of His glory and the express image of His person, and upholding all things by the word of His power, when He had by Himself purged our sins, sat down at the right hand of the Majesty on high,

Sam Shamoun comments on this text: "There are several points which we can glean from this very crucial text. The first point is that Jesus is the very exact imprint, the very exact copy, the perfect reflection of God’s own substance, nature, essence etc. That is the meaning of the Greek word charakter, that Jesus is the precise and perfect imprint left by the Original or the Source. The author of Hebrews is basically saying that the Father is the underived Source of all Deity with the Son being the perfect duplicate of that Deity. If God’s substance is eternal, then Christ must be eternal also since he is the exact imprint. If God’s substance is infinite, then Christ must also be infinite seeing that he is the exact copy of it."

A similar point is made in Colossians 1:15.

It is also implied by the fact that Jesus Christ is the Son in eternity, we see Jesus called the Son already in Proverbs 30:4, before the incarnation, which states:

Who has ascended into heaven and descended? Who has gathered the wind in His fists? Who has wrapped the waters in His garment? Who has established all the ends of the earth? What is His name or His Son’s name? Surely you know!