Although Gnosticism in its fully-developed form dates from the late 2nd and 3rd centuries AD, its roots go back much earlier. Some of Paul's statements in his later letters makes a lot more sense when read against the backdrop of incipient Gnosticism, in which case its first manifestations came in the early AD 60's, but it was the apostle John had to mount the first proper defense against its teachings and its acceptance into church circles. One of the main early proponents of Gnosticism, Cerinthus, was active in Ephesus at the same time John was ministering there, and the 4th century church father Jerome states that John "most recently of all the evangelists wrote a Gospel, at the request of the bishops of Asia, against Cerinthus and other heretics and especially against the then growing dogma...that Christ did not exist before Mary. On this account he was compelled to maintain His divine nativity".

By the time John wrote his first letter the Gnostic heresy was much more developed, and the need for a strong defence against it was much greater. Rather than naming names and personally attacking those spreading this false teaching, John powerfully presents the Christian truth against the false teachings of Gnosticism. Notably, Gnosticism's main "appeal" was secret knowledge, but John undercuts it by hammering on the word "know", often stating "this is how we know", or "by this we know". (The word "know" appears 32 times in this short letter, "known" appears twice.)

To people being tempted by "secret knowledge" – then, as well as in our day – John is saying "But you have an anointing from the Holy One, and all of you know the truth. I do not write to you because you do not know the truth, but because you do know it and because no lie comes from the truth" (1 John 2:20,21). And he warns them "see that what you have heard from the beginning remains in you. If it does, you also will remain in the Son and in the Father" 1 John 2:24,25.

Without explicitly stating it, John opposes five of its core tenets by very clearly setting out the gospel truth for his readers (and hearers).

1. REJECTION OF THE OLD TESTAMENT GOD

The creator God of the Old Testament was often identified in Gnosticism as the ignorant or malevolent Demiurge, distinct from the true, supreme God who was utterly transcendent and unknowable without special revelation (knowledge). This Demiurge was a lesser divine being, one of a sequence of divine intermediary beings emanating from the Supreme Being, a kind of "craftsman" who created the material world (and made a mess of it). He was viewed as inferior to the supreme, transcendent God, and is a prominent figure in Gnostic cosmogony, sometimes depicted as ignorant, malicious, or simply well-meaning but limited, depending on the specific Gnostic tradition.

Against this perversion of God John presents the one true God:

God is not unknowable, we can have fellowship with Him

1 John 1:3: We proclaim to you what we have seen and heard, so that you also may have fellowship with us. And our fellowship is with the Father and with his Son, Jesus Christ.

God is light [goodness] no darkness [evil] in Him

1 John 1:5: This is the message we have heard from him and declare to you: God is light; in him there is no darkness at all

Links knowledge of Father and Son to eternal life, not secret knowledge. Enlightenment comes not through secret knowledge, but only through God

1 John 2:24,25: As for you, see that what you have heard from the beginning remains in you. If it does, you also will remain in the Son and in the Father. And this is what he promised us – eternal life

Only the love of God enables the "divine spark" in each human reach its true destiny

1 John 3:1-3: See what great love the Father has lavished on us, that we should be called children of God! And that is what we are! The reason the world does not know us is that it did not know him. Dear friends, now we are children of God, and what we will be has not yet been made known. But we know that when Christ appears, we shall be like him, for we shall see him as he is. All who have this hope in him purify themselves, just as he is pure.

God is not an evil, mean intermediary, a hindrance on the way to knowledge – He is the transcendent and unknowable Supreme Being who was revealed to us in Jesus Christ, and loves us dearly.

1 John 4:9,10: This is how God showed his love among us: He sent his one and only Son into the world that we might live through him. This is love: not that we loved God, but that he loved us and sent his Son as an atoning sacrifice for our sins. (See John 1:18: No one has ever seen God, but the one and only Son, who is himself God and is in closest relationship with the Father, has made him known.")

2 DENYING THE TRUE NATURE OF CHRIST

Flowing from its dualistic view of matter, Gnostics struggled with the concept of God taking on a physical body.
Many adopted Docetism (from the Greek dokeo, "to seem"), which taught that Jesus only appeared to have a physical body, but was in reality a spirit. He did not truly suffer or die.
The other option was Cerinthianism (developed in Epesus by Cerinthius), the view that the "Christ spirit" descended upon the man Jesus at his baptism and departed before the crucifixion. It separates the divine Christ from the human Jesus, suggesting Jesus was a mere man, the natural son of Joseph and Mary, teaching that the divine Christ (or Logos) joined Jesus at his baptism and departed just before his crucifixion.

John vehemently asserts that Jesus was fully human by stressing the eyewitness testimony of the apostles, who touched, heard, and saw the incarnate Word, and rejected Gnostic attempts to separate the divine "Christ" from the human "Jesus," or to deny Jesus's unique identity as the Son of God.

Jesus was a physical human who could be experienced tangibly when He was on earth

1 John 1:1-3: "That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, concerning the Word of life - the life was manifested, and we have seen, and bear witness, and declare to you that eternal life which was with the Father and was manifested to us - that which we have seen and heard we declare to you, that you also may have fellowship with us; and truly our fellowship is with the Father and with His Son Jesus Christ."

John gives a direct test for orthodox belief, explicitly condemning Docetism

1 john 4:2,3: This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world.

Belief in Jesus as the Christ is foundational to being born of God

1 John 5:1: Everyone who believes that Jesus is the Christ is born of God, and everyone who loves the Father loves his child as well.

John refers to both Christ's baptism and His crucifixion, affirming the physical reality of His life and death

1 John 5:6: This is the one who came by water and blood – Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth

He clearly indentifies those who deny Christ's identity as antichrists

1 John 2:22,23: Who is the liar? It is whoever denies that Jesus is the Christ. Such a person is the antichrist – denying the Father and the Son. No one who denies the Son has the Father; whoever acknowledges the Son has the Father also.

He reminds them that faith in Jesus as the Son of God is the means of overcoming - not knowledge

1 John 5:5: Who is it that overcomes the world? Only the one who believes that Jesus is the Son of God.

3. SALVATION THROUGH ESOTERIC (SECRET) KNOWLEDGE

According to Gnosticism salvation was not through faith in Jesus's physical death and resurrection but through gaining secret, mystical knowledge about one's true divine origin ("divine spark") and how to escape the material world. This knowledge was often reserved for an enlightened few – Gnosticism identified three types op people: the spiritual (who could achieve gnosis), the psychic (who might be saved through faith and good works), and the material (who were hopelessly bound to the physical world).

John consistently links true knowledge of God with obedience to His commandments and a recognition of one's own sinfulness, rather than a secret, intellectual ascent. He condemns those who claim to know God but live in darkness or deny their sin.

True knowledge (understanding) comes only through Christ

1 John 5:20: We know also that the Son of God has come and has given us understanding, so that we may know him who is true

True knowledge, even of us, comes from God, nowhere else

1 John 3:19,20: This is how we know that we belong to the truth and how we set our hearts at rest in his presence: If our hearts condemn us, we know that God is greater than our hearts, and he knows everything

Salvation comes not though esoteric knowledge, but through Jesus Christ alone

1 John 1:1-2: My dear children, I write this to you so that you will not sin. But if anybody does sin, we have an advocate with the Father – Jesus Christ, the Righteous One. He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world.

Knowledge of God is demonstrated by obedience, not secret insight

1 John 3:10: This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not God's child, nor is anyone who does not love their brother and sister.
1 John 2:5b,6: This is how we know we are in him: Whoever claims to live in him must live as Jesus did.
1 John 2:3-4: We know that we have come to know him if we keep his commands. Whoever says, "I know him," but does not do what he commands is a liar, and the truth is not in that person.

Salvation comes not through esoteric knowledge, but through doing what is right – contrasting with the Gnostic idea the irrelevance of morality

1 John 3:10: This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not God's child, nor is anyone who does not love their brother and sister.

John connects fellowship with God with walking in light and acknowledging sin

1 John 1:5-7: This is the message we have heard from him and declare to you: God is light; in him there is no darkness at all. If we claim to have fellowship with him and yet walk in the darkness, we lie and do not live out the truth. But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin.

We have knowledge of and fellowship with God through his Spirit, not secret knowledge

1 John 3:24: And this is how we know that he lives in us: We know it by the Spirit he gave us.

4. DENIAL OF SIN

According to Gnosticism, since the body and material world were inherently evil, then actions performed in the body might be deemed irrelevant to spiritual purity. This could lead to antinomianism, where moral laws were disregarded, or to extreme asceticism, where the body was severely disciplined to "free" the spirit.

John emphasises strongly that sin is real, and have serious consequences if not dealt with correctly

Knowledge that denies the reality of sin is not truth

1 John 2:4: Whoever says, "I know him," but does not do what he commands is a liar, and the truth is not in that person.

John defines sin as a breaking of God's law, contrasting with the Gnostic idea of moral irrelevance

1 John 3:4-6: Everyone who sins breaks the law; in fact, sin is lawlessness. But you know that he appeared so that he might take away our sins. And in him is no sin. No one who lives in him keeps on sinning. No one who continues to sin has either seen him or known him.

John states that the Gnostic denial of the reality of sin is not truth (true knowledge)

1 John 1:8-10: If we claim to be without sin, we deceive ourselves and the truth is not in us. If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. If we claim we have not sinned, we make him out to be a liar and his word is not in us.

Sin is real, and we should not live in it but in Christ

1 John 1:7 & 2:5b,6: But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin... This is how we know we are in him: Whoever claims to live in him must live as Jesus did.

5. DEVALUATION OF COMMUNITY (FELLOWSHIP) AND CHURCH AUTHORITY

In Gnosticism salvation was an individualistic pursuit of knowledge, focussed on individual spiritual ascent, which supposedly came through direct spiritual experience. This undermined the importance of communal worship, established church leadership, and apostolic teaching.

In contrast to this individualistic pursuit John strongly emphasizes love for one another as a non-negotiable sign of genuine Christian life and fellowship with God

Jesus is the supreme example, demonstating love to us - he doesn't love in isolation, and neither can we

1 John 3:16: This is how we know what love is: Jesus Christ laid down his life for us. And we ought to lay down our lives for our brothers and sisters.

Love for fellow-believers is a mark of a child of God

In fact, love for our fellow-believers is a test for walking in the light (truth)

1 John 2:6, 9-11: This is how we know we are in him: Whoever claims to live in him must live as Jesus did...Anyone who claims to be in the light but hates a brother or sister is still in the darkness. Anyone who loves their brother and sister lives in the light, and there is nothing in them to make them stumble. But anyone who hates a brother or sister is in the darkness and walks around in the darkness. They do not know where they are going, because the darkness has blinded them.

John links love for others directly to knowing God and being born of God

1 John 4:7-8: Dear friends, let us love one another, for love comes from God. Everyone who loves has been born of God and knows God. Whoever does not love does not know God, because God is love

There is an inseparable link between loving God (having true knowledge) and loving others

1 John 4:20-21 Whoever claims to love God yet hates a brother or sister is a liar. For whoever does not love their brother and sister, whom they have seen, cannot love God, whom they have not seen. 21And he has given us this command: Anyone who loves God must also love their brother and sister.

1 JOHN AND GNOSTICISM AT THE CLOSE OF THE FIRST CENTURY