Thank you for the question, “Was the person who wrote the Gospel of John a gnostic leader?
Gnosticism can be defined as the doctrine that the world was created and ruled by a lesser divinity or a series of lesser divinities of whom Christ was one of them. In other words, Gnosticism teaches that Jesus Christ is NOT God himself but a created being liken to an angel.
As one reads the Gospel of John, the Apostle John does not teach that Jesus is less than God, but that he is God incarnated. He affirms that Jesus is God. I cite for one’s consideration two examples from the Gospel of John.
A. John 1:1–3.
The NIV text says this, “In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was with God in the beginning. 3 Through him all things were made; without him nothing was made that has been made.
As I read John 1:1 in the Greek and English text, the Apostle John states that the Word or Logos was present from the beginning. In the Greek text, there is the absence of the definite article (the). The text literally says, “In beginning was the logos,”. One can add the definite article but by adding the definite article it may imply that the logos was not present before the beginning (of creation). Without the definite article, it may imply infinite pass or self-existence before creation.
The Apostle John states further that the Word (logos) was with God. The implication is that if the first part of the statement of his eternal existence is true then the Word was always with God. There was not a time in which the Word was not with God.
The Apostle John further states that the Word was God. John does not say that the Word “became” God. If John wanted to say that the Word was created, the word, became, which would imply a beginning, not self-existed, would have been used. He does not choose that word, became, but instead the verb of being, was. It is also important for one’s consideration that John does not say that the Word was the Father. He makes a distinction between the Word and the God.
If John placed an article, the, before God, it would say this: “And the Word was THE God. The implication is that the Word and the God are the same person or one person with two names. This is not the case as John distinguishes between the Word and God himself.
John further states in verse 2 that the Word was with God in the creation of the world. God didn’t create the Word. The world was created through the Word. Nothing was made apart from him (vs. 3). Jesus is not a lesser divine being, but God himself being revealed as the “Second Person” in the Godhead.
B. John 8:49–59.
The text says this, “The Jews answered him, “Aren’t we right in saying that you are a Samaritan and demon-possessed?” 49 “I am not possessed by a demon,” said Jesus, “but I honor my Father and you dishonor me. 50 I am not seeking glory for myself; but there is one who seeks it, and he is the judge. 51 Very truly I tell you, whoever obeys my word will never see death.” 52 At this they exclaimed, “Now we know that you are demon-possessed! Abraham died and so did the prophets, yet you say that whoever obeys your word will never taste death. 53 Are you greater than our father Abraham? He died, and so did the prophets. Who do you think you are?”
54 Jesus replied, “If I glorify myself, my glory means nothing. My Father, whom you claim as your God, is the one who glorifies me. 55 Though you do not know him, I know him. If I said I did not, I would be a liar like you, but I do know him and obey his word. 56 Your father Abraham rejoiced at the thought of seeing my day; he saw it and was glad.” 57 “You are not yet fifty years old,” they said to him, “and you have seen Abraham!” 58 “Very truly I tell you,” Jesus answered, “before Abraham was born, I am!” 59 At this, they picked up stones to stone him, but Jesus hid himself, slipping away from the temple grounds” (NIV).
Jesus states that the Patriarch Abraham not only thought about seeing my (Jesus) day and he SAW it and was glad. The Jews did not challenge Jesus’ statement and argues that how can he have seen Abraham! Jesus isn’t hundreds of years old is he?
Jesus statement was quite surprising or shocking to the Jewish leaders. Jesus said, “Before Abraham was born, I am (AM)!” He existed before Abraham existed. When Jesus said that “I AM” implies and refers back to YAHWEH who revealed himself to Moses as “I AM WHO I AM.” It is interesting to note that Jesus didn’t refer to the burning bush experience with Moses but to the founder of the Jewish faith, Abraham. Abraham refers to YHWH (I AM).
Genesis 14 says this, “But Abram said to the king of Sodom, “With raised hand I have sworn an oath to the LORD, God Most High, Creator of heaven and earth, 23 that I will accept nothing belonging to you, not even a thread or the strap of a sandal, so that you will never be able to say, ‘I made Abram rich.’ 24 I will accept nothing but what my men have eaten and the share that belongs to the men who went with me—to Aner, Eshkol and Mamre. Let them have their share.” (NIV).
It appears that Abram was aware of the LORD, who is God Most High, Creator of heaven and earth. Was Abram or Abraham aware that he was speaking to the LORD God? I believe so. The point is this that Jesus is referring that he is the LORD, God Most High, Creator of heaven and earth.
The religious sought to stone Jesus for blasphemy, claiming to be LORD God. No Jew would dare make that kind of claim, but Jesus did. He is the Creator of heaven and earth.
As one reads the Gospel of John, read carefully the words of Jesus and what do you believe about what he says about himself.
SUMMARY: the writer, Apostle John was not an Gnostic. John argues for the deity of Jesus Christ as the God-Man, not a created divine being who became God.
https://www.quora.com/Was-the-person-who-wrote-the-Gospel-of-John-a-gnostic-leader