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Jesus existed in the beginning?
John 1:1-2 simply states this when it says
In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God.
And who was the Word? Just a few verses later John clearly tells us
And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth.
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 1:14-15)
Who was it that became flesh to dwell among us, the only begotten of the Father (John 3:16)? Who was it that John bore witness to? It was Jesus. Jesus existed in the beginning with God, and not just existing with God, but Jesus was God in the beginning! (He is still God today of course.

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Isaiah the prophet heard a message from God where God the Father said
Thus says the LORD, the King of Israel, And his Redeemer, the LORD of hosts: ‘ I am the First and I am the Last; Besides Me there is no God. (Isaiah 44:6)
God is the First and the Last, nothing existed before God and nothing will exist after Him, He is it, the first and the last. In Revelation Jesus identifies himself as ” I am the Alpha and the Omega, the Beginning and the End, the First and the Last.” (Revelation 22:13) The Alpha and Omega are the first and last letters of the greek alphabet, in English it would be the A and the Z. There are no letters before A, and none after Z. Jesus is the first and last, the beginning, and the end.
How is it that both God the Father and Jesus can be the first and the last? This is only possible because Jesus is God. Remember what John 1:1 said, “The Word was with God, and the Word was God.” Both are God, both are one being, one diety, even though they are individual.