Pre-existence of Christ
The pre-existence of Christ is the belief that Christ did exist before he was even born, that he was the fully divine person God the Son until he chose to be "made flesh" to then take over two natures: divine and human.

John 1:15 reads:
John bore witness of Him and cried out, saying, “This was He of whom I said, ‘He who comes after me is preferred before me, for He was before me.’”
— John 1:15
In John 17:5, Jesus talks about the glory that he had with the Father "before the world existed" and in verse 24, that the Father loved him "before the foundation of the world".[2]
John 1 opens with "In the beginning was the Word, and the Word was with God..." this "Word" is later said to have been Jesus in verse 14 but just "made flesh".
Nicene Christianity
The pre-existence of Christ is one of the most important beliefs in mainstream Christianity. Most mainstream churches that accept the Nicene Creed view the nature of the pre-existent Christ as the fully divine person called the Word, which is described in John 1:1-3; the verses read:
In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was in the beginning with God. 3 All things came into being through him, and without him not one thing came into being.
— John 1:1–3, NRSV