In the opening verses of the Gospel according to John, we learn about two cousins. One was the Word or Light. This is Jesus. The other was witness to that Light. This is John the Baptist. The first three verses present a lot of doctrine about the eternal nature of Jesus and the Trinity.
In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him; and without him was not any thing made that was made. – John 1:1-3
John clearly teaches that Jesus has always existed. Jesus was already in existence at the creation of our universe. And not only was Jesus not created, He created everything else!
John also teaches us some details about the mystery of the Trinity. He says that Jesus was both with God and was God. He then repeats that He was with God. John leaves no doubt that he understood the godhead to be plural in number. Contrary to what more liberal theologians believe, this concept of plurality was not created in Rome several hundred years after Christ. It was a concept well understood by true students of the Old Bible among the Jews.
The very first book of Moses, Genesis, speaks of this plurality within God in the opening pages. The name of God, Elohim in Genesis 1:1, is the plural of Eloha. In 1:26, Moses writes “Let us make man in our image, after our likeness.” The God of Creation had a plurality within Himself. John simply tells us Who One of those persons is!
Jesus did not come into existence when He was born of Mary. John testifies in verse 14 that the Word was made flesh. This means the Word (Jesus) already existed.
We can find Jesus in the pages of the Old Testament on multiple occasions. There is clear evidence that Jesus appeared preincarnate as the “angel of the Lord” to Hagar and Abraham. Paul tells us in 1 Corinthians 10:4 that Jesus was the Rock from which the Israelites drank in the wilderness. And one my favorites is Jesus’ appearance as “the captain of the host of the LORD” in Joshua Chapter 5. These are just a few of the many clear appearances of Jesus in the Old Testament proving He is the eternal God.
John concludes his opening in verse 18:
No man hath seen God at any time, the only begotten Son, which is in the bosom of the Father, he hath declared him.
Basically what he is telling us is that anytime anyone saw God, they were seeing Jesus. Jesus has the fullness of the godhead in Himself. Everything we need to know about God we find in Jesus Christ. And here is the awesome blessing, Jesus has told us in the gospels all we need to know about Him and what He did for us!