While there are implications from various verses (i.e. compare God [Is 44:6] and Jesus [Rev 22:12-13]), the idea that Jesus existed before Creation is derived explicitly from John 1:1-3.

In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things came into being through Him, and apart from Him nothing came into being that has come into being. (John 1:1-3)

Writing in the Jewish fashion of using imagery and contrast, the apostle John uses a metaphor for Jesus. Jesus is the Word, which is seen when the apostle explicitly explains a few verses later:

And the Word became flesh, and dwelt among us, and we saw His glory, glory as of the only begotten from the Father, full of grace and truth. John testified about Him and cried out, saying, "This was He of whom I said, 'He who comes after me has a higher rank than I, for He existed before me.'" (John 1:14-15)

What is the apostle pointing out when calling Jesus the Word?

In contrast to Old Testament prophets, Jesus never once prefaced His message with "thus says the Lord" or "the word of God came to me."

As the last Old Testament prophet, John the Baptist (Luke 3:2) recognized Jesus as the Messiah, "For He whom God has sent speaks the words of God; for He gives the Spirit without measure. The Father loves the Son and has given all things into His hand." (John 3:34-35)"

Jesus Himself expresses that all authority has been given to Him (Matt 11:27; Luke 10:22; John 6:37-39; 13:3; 17:2).

This becomes more apparent in the Sermon on the Mount when Jesus says, "You have heard that the ancients were told…" (Matt 5:21, 27, 31, 33, 38, 43), "But I say to you…" Here Jesus broadens the meanings of past statements which were considered divine commands and amends it with His own "I" in the place where Yahweh would otherwise be expected. In so doing, Jesus accomplishes two things: 1) He sets Himself against the Pharisees' understanding of the Law, and 2) He establishes the legitimacy of His words alongside of the God's word and by extension God Himself.

The apostles never prefaced their message with "thus says the Lord" or "the word of God came to me." Instead they bore witness of Jesus Christ's life and death and cited what Jesus said as authoritative as God's word.

John 1:1 is a significant opening for the book of John, because it makes 3 defining statements about the Word.

"In the beginning was the Word,.." (John 1:1)

When the apostle John opens his introduction with "in the beginning," there is a reference to the beginning of Creation:

In the beginning God created the heavens and the earth. (Gen 1:1)

However, rather than to suggest that Jesus was created at Creation, the Greek phrase for "in the beginning was the Word" literally should be translated as, "when the beginning began, the Word was already there." The apostle John was clearly stating that Jesus existed before Creation.

"... and the Word was with God,.." (John 1:1)

The apostle John's Greek phrase here conveys the idea that the Word is equal to and distinct from God. The context of the Greek phrase implies a fellowship and coexistence with God.

"… and the Word was God." (John 1:1)

The apostle states clearly to the deity of the Word. Associating the article "the" with "Word" establishes "the Word" as the subject of the phrase, and "God" is describing the nature of "the Word." The Word is the same nature of God, divine yet a separate person.

Following his opening verse (John 1:1), the apostle John restates and makes clear his meaning:

He was in the beginning with God. All things came into being through Him, and apart from Him nothing came into being that has come into being. (John 1:2-3)

Without a doubt, the apostle intended his audience to know that Jesus was present at the time of Creation. Moreover, Jesus played an essential role in Creation.