For this series, I am using my own translation of 1, 2, and 3 John.
Eyewitness
John probably wrote this letter fifty years after Jesus had ascended into heaven, so John began by reminding his readers about the reality of Jesus, that he, John, had been an eyewitness to the amazing events of Jesus’ time on earth, and that John had not only seen Jesus transfigured, but also resurrected from the dead.
The One Who was from the beginning,
Whom we have heard, Whom we have seen,
our eyes beheld Him, our hands touched Him,
concerning the Word of life—and the life was made visible, and we have seen and bear witness and declare to you
the [eternal life] Who was from the Father and made visible to us—we have seen and heard Him, and we bear witness to you,
John 1:1-3

Note the physicality of John’s experience—he saw, heard, and touched, establishing Jesus as both from the earth as a physical human being, and from God as the Son.
Also note the two parallelisms. John wrote this with poetry in mind, Greek in language but Jewish in heart.
As a witness, Jesus’ disciple, and now an apostle, John had the credentials to proclaim the truth, bringing his readers into fellowship with him and with God, completing their joy.
and in order that you have fellowship both with us and with the Father and with the Son Himself, Jesus Christ.
And we write these things for us [all] in order that our joy [together] may be filled full.
1 John 1:3-4
Fellowship was not just between John and those to whom he was writing, fellowship included this supernatural component, both physical and spiritual connection with the Father and with the Son. This is the mark of Christian fellowship, the completion of joy that only comes with sharing the things of Christ among each other.
Fellowship and truth are key concepts in this letter, as are abiding in Christ, the contrast between light and darkness, between life and death, between love and hate, the contrast between assurance and doubt and fear, between righteousness and worldliness, and between obedience and error.
And this is the message we have heard from Him and declare to you, that God is light and no darkness at all is in Him.
1 John 1:5
John began with the truth about God. God is light, signifying truth, goodness, joy, safety and life, as contrasted with darkness, which is wrong thinking, evil, and death. In order to be in the light, believers have to understand the truth
If / Then
John then countered the three main claims of the false teachers, and he did it with a series of “if” statements that imply a “then.”
IF we said that we have fellowship with [Jesus] and walk around in the darkness, we are lying [to] ourselves and we do [not live the truth],
And IF we walk around in the light, as He is in the light, we have fellowship with each other, and the blood of Jesus His Son cleanses us from all sin.
IF we say we have not sin, we deceive ourselves and the truth is not in us.
IF we [are acknowledging of] our sins, trustworthy and righteous [is the One] Who forgives us and cleanses us from all unrighteousness.
IF we say we do not trespass, we make Him a liar and His word is not in us.
1 John 1:6-10
First Claim of the Gnostics
1:6, It is impossible to claim fellowship with God, Who is light, when actually living in the dark. That is the nature of darkness, that it never, ever stays dark when the light shines in. It is impossible. Light always gets the better of darkness. It is impossible to be in darkness yet claim one is in the light. The two cannot coexist.
What was John really saying? Denigrating the body either through mortification or licentious indulgence is darkness. Living a lawless life, a life apart from God’s guidance and instruction, is to not have fellowship with God.
The counter claim, found in 1:7, is not to perfectly obey the law, but to live already as one made sacred, as one who is already cleansed and made pure.
Second Claim of the Gnostics
1:8, Claiming to be without sin is an impossible claim. Sin is just one of those things, it is a catch phrase that includes all the faults, frailties, and failings of being human. All people have sinful nature, because all people are human!
John reminded his readers that if we say we have no sin, then we deceive ourselves. Everybody has the same condition. Sin is a condition every person—whether by nature or by nurture—experiences. Everyone is raised in a family that may have its good points, but certainly also has its shortcomings. And even if one’s family was perfect, one’s environment certainly is not. There will always be pressures and influences that mold us in good and bad ways.
So!
We deceive ourselves if we try to say we have no sin. It just is not true.
Consequently, because sin is something that all people have in their natures, everyone has also engaged in some sort of sin, whether large or small. And sin, a priori, is to fall short of God’s glory, God’s nature and character.
We are far better off to just come clean with God (and ourselves, for that matter) and confess that we do have sin because . . . we really do.
Cleansed From All Unrighteousness
The counterclaim is not to self-flagellate, or to claim responsibility for failures that really are someone else’s, or to labor under guilt and shame. But rather, to come to God and receive God’s cleansing.
1:9, This is one of my most favorite verses in all the scriptures. I place it side-by-side with Joseph’s statement to his brothers and God’s promise made through Paul. Here they all are together.
Even though you intended to do harm to me, God intended it for good, in order to preserve a numerous people, as he is doing today.
We know that all things work together for good for those who love God, who are called according to his purpose. For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn within a large family. And those whom he predestined he also called; and those whom he called he also justified; and those whom he justified he also glorified.
If we [are acknowledging of] our sins, trustworthy and righteous [is the One] Who forgives us and cleanses us from all unrighteousness.
Joseph to his brothers, Genesis 50:20 (NRSV); the Apostle Paul, Romans 8:28-30 (NRSV); 1 John 1:9
God is at work all around us and within us. There are those who harm us, violate us, manipulate and abuse us, dump their mess on us, leave us holding the bag. There are those who through gaslighting, or profound neglect, or sadistic cruelty, leave terrible marks on us. Faulty as we may be, there are times when we really are the victim, and it is not our fault.
And, we have to face it, there are times when it is we who have done the harm or left the mark. Perhaps intentionally, perhaps in ignorance, but every person has also used their agency to do wrong by another.
But God is also at work. In and through the caprices of human choices God still sovereignly guides towards salvation and sanctification. God is able to grant free will and still work what happens towards eternal good, the saving of many lives, justified and glorified, cleansed of all unrighteousness.
That is the incredible mighty wonder-working power of God, that when we confess, God will cleanse.
This is not simply about “fessing up,” this is about both sides of the sin equation, what has been done to me as well as what I have done. For what has been done to me is another’s unrighteousness left to fester in my own heart, soul, and spirit. This too God promises to cleanse as we bring it all up before the Lord and receive God’s sorrow for us, God’s outrage over the wrong, and God’s powerful life-giving love.
Third Claim of the Gnostics
Is actually found at the beginning of chapter 2.
[ascension of Jesus | By James Tissot – Online Collection of Brooklyn Museum; Photo: Brooklyn Museum, 2006, 00.159.349_PS1.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=10957950%5D