The Preexistence of the Son
The Son of God has always existed, just as God has always existed. Being Yahweh as to His nature, the Son has always been with the Father. The Father's glory has always been shared with the Son. The following portion of Christ's last prayer before His betrayal reveals His relationship with the Father Whom He had been progressively revealing.
5Now, Father, glorify me with your own self with the glory which I had with you before the world existed. 24Father, I desire that they also whom you have given me be with me where I am, that they may see my glory, which you have given me, for you loved me before the foundation of the world. John 17:5,24
Before Abraham was, the Lord Jesus is.
58Jesus said to them, “Most certainly, I tell you, before Abraham came into existence, I AM.” John 8:58
Even before the world existed, this Son was!
1In the beginning was the Word, and the Word was with God, and the Word was God. 2The same was in the beginning with God. John 1:1-2
The apostle John takes us back to the creation story, where it is written, "In the beginning, God..." (Genesis 1:1) Here he continues the revelation which came through Christ Jesus, that, "In the beginning, the Word..."
As he reveals, the Word was both with God and was God. He was Divine as to His nature, as there is no article before "God" in the Greek ("και θεος ην ο λογος" - literally, "and God [Divine] was the Word"). Not that He was all there was to God, i.e., not "the God," (as there also is the Father and the Holy Spirit within that same God, Yahweh), but that He is Divine, He is Deity. He was ever present with God, the Father. He is Yahweh just as is the Father.
It is important to begin with this truth, because in understanding the Son better, we come to know Him better, to be joined more closely to Him. In addition, through seeing the revelation of Yahweh concerning His Son, we are protected from "other Gospels, which are not Gospels," and from "another Jesus," who is not Jesus. For there are many testimonies about this Son, that do not come from the Father—which claim Him to be only a prophet, only a holy man, only a teacher, only an anointed one, and not, as He truly is, God in the flesh.
16Without controversy, the mystery of godliness is great: God was revealed in the flesh, justified in the spirit, seen by angels, preached among the nations, believed on in the world, and received up in glory. 1 Timothy 3:16
Demonic Dispute
As Paul immediately says after sharing this great mystery, many will depart from the faith, cleaving to doctrines of demons (1 Timothy 4:1). For there are many teachings today which deny that Jesus is Yahweh, that He is not God/Divine, just as prophesied would happen. But by clinging to the revelation the Father Himself has given through His Son, we might be kept safe from such demonic doctrines.
19We know that we are of God, and the whole world lies in the power of the evil one. 20We know that the Son of God has come, and has given us an understanding, that we know him who is true, and we are in him who is true, in his Son Jesus Christ. This is the true God, and eternal life. 1 John 5:19-20
By understanding the Son's preexistence with the Father, we come to appreciate Him as sharing all that the Father has. Because He has always been, so He may bring us all that the Father may bring. In Him we find all the goodness of God,
9For in him all the fullness of the Deity dwells bodily, Colossians 2:9
The Preexistence as Power to Save
The glory of the Son's eternality alongside the Father is that He has chosen to come to us from that high place. He did not remain in heaven to only be worshipped, but humbled Himself to be born the Son of Man, to save us and bring us into the presence of Yahweh.
5Have this in your mind, which was also in Christ Jesus, 6who, existing in the form of God, didn’t consider equality with God a thing to be grasped, 7but emptied himself, taking the form of a servant, being made in the likeness of men. 8And being found in human form, he humbled himself, becoming obedient to death, yes, the death of the cross. Philippians 2:5-8
Paul reveals that Christ was "existing in the form of God." As John says, the Word was both with God and was God. He had the very form of God because He was the Son of God—whatever is begotten of God shares the nature of God. Therefore, in His final priestly prayer, He asks for the same unity that He and the Father share to be shared by His people. Just as they are one so He wants us to be one. And this unity comes through our unity with Them (John 17:20-23).
Because the Son of God has always existed, alongside and facing the Father, so we can trust Him in all things. For as He said,
1“Don’t let your heart be troubled. Believe in God. Believe also in me. John 14:1