Within a proper understanding of Christology one must come to terms with the difference between the eternity of the Son and the pre-existence of the Son. That is one speaks of His work prior to His birth while the other speaks of His work throughout eternity, both past and future. The eternity of the Son is associated with His eternal position in the Godhead from eternity past to eternity future. Jesus, praying to the Father asks that the redeemed be able to see Him in the Glory that He had “before” the foundation of the world (John 17:24). Paul speaks of His eternality in His choosing us before the foundation of the world (Eph 1:4). Peter tells us that the Lamb of God (the Son, Jesus) was foreordained before the foundation of the world to shed His blood for the redeemed (1 Peter 1:20). Each of these present both past and future aspects of His work as it relates to man in that the pre-existence of the Son refers to His pre-incarnate ministry before His birth in Bethlehem. This would be His work in the Old Testament. This of course offers evidence to His eternity.
So then how is Jesus eternal?
Strictly speaking, Jesus’ eternality began with His birth whereby Paul writes quoting the Psalms in Heb 2:7-9 that Jesus was made a “little lower than the angles” and is now seated at the “right hand of God” (Col 3:1). (cf. Matt 26:64; Acts 7:55; Rom 8:34; Peter 3:22 et al.) Upon this occasion deity became flesh and dwelt among us Jn 1:14. His being fully God and fully man in no wise diminishes one another. Although His glory and other attributes were voluntarily veiled (in flesh), He was nonetheless God!
It is therefore not correct to speak of Jesus before His incarnation. Again strictly speaking the work of Jesus began in
when the Word clothed Himself in flesh (Jn 1:14). This denotes a particular facet of His eternal work for the redemption of man whereby God (the Father) made a covenant with His Son in eternity past. That is before the foundation of the world God (including the Son) foreordained that Christ would die for the ungodly. This was to display the importance from eternity past, even before Adam sinned, the redemption of man to His glory. Bethlehem