What follows is a verse-by-verse explanation of the book of first Chapter of 1 John. Feel free to comment, or add your own insight, as I am certain I will not glean every grain of truth from this wonderful book, nor would I expect to; one of the best things about God's word is that it is alive, and the Holy Spirit can reveal new things each time we read through it. I will do my best to be as accurate as I can with definitions and explanations, but there is always room for growth, so feel free to do your own study along with me, and decide for yourself what John, by the Holy Spirit, is telling us in his first epistle to the church of Christ.
I will be citing primarily the King James bible; I prefer it, for one thing, because it is a literal translation, versus paraphrased (not to say it is the only reliable translation, nor completely without flaw).
THE FIRST EPISTLE OF JOHN
Written by the apostle John, about AD 85-AD 95, to the church in general.
1Jo 1:1 KJV - 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, of the Word of life;
1Jo 1:2 KJV - (For the life was manifested, and we have seen it, and bear witness, and shew unto you that eternal life, which was with the Father, and was manifested unto us;)
John testifies to being an eye witness of Christ's life on earth. He was there for Christ's ministry, the time leading up to his death, and saw him after his resurrection. (See also the Gospel of John 1:1-4, where John also refers to Christ as being the Word)
1Jo 1:3 KJV - That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.
1Jo 1:4 KJV - And these things write we unto you, that your joy may be full.
John's purpose in imparting this Spiritual knowledge to the church is (1) to bring unity within the body, (2) thus bringing unity between the church and God, (3) to bring to the church fullness of Spiritual joy.
Strong's definition of "full" G4137: "to make replete, i.e. (lit.) to cram (a net), level up (a hollow), or (fig.) to furnish (or imbue, diffuse, influence), satisfy, execute (an office), finish (a period or task), verify (or coincide with a prediction), etc.
1Jo 1:5 KJV - This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all.
This, you could say, is the basis of John's entire letter. God is light, and there is no darkness in him. It is important to note that "in" does not simply mean "inside of", as in "God has no sin within himself", which, while that is true, the following verses suggest that "in" implies "in relation to".
1Jo 1:6 KJV - If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:
God who is light (used to symbolize purity, clarity, understanding, etc.), will not dwell in the presence of someone who is dark (symbolically impure, obscure, and without understanding). So if we, the proclaiming believers, are walking in darkness (i.e. sin) but calling ourselves Christians, we are liars.
1Jo 1:7 KJV - But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.
If we are following the light that God's word casts on the ground before our feet (walking according to His commandments), then we are in unity with the rest of the body, and our old sins are not credited against us, because, as Christians, we have repented of and confessed our sins, and through the blood of Jesus Christ, they have been washed away.
1Jo 1:8 KJV - If we say that we have no sin, we deceive ourselves, and the truth is not in us.
We must take this verse in context with all of what John is saying. His point thus far has been that, if we claim to have a relationship with God when we are in darkness, we are NOT in a relationship with God at all. In verse 8 John is not saying that we all have sin within us presently (because he just said in v 7 that the blood of Christ has cleansed us from our sins), rather he is emphasizing the point: if we say we don't have sin when we do, then we are liars.
So how do we not have sin within us?
1Jo 1:9 KJV - If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
Note that 1 John is written to the church, not to the world, so this verse is not referring to our initial salvation, but to the need for repentance if we sin after we are saved. And if we do repent for our sins, God is faithful to forgive us and to cleanse us from all unrighteousness. At this point, it would not be a lie to say "I walk in the light, I have no sin". We cannot separate our body from our spirit in the sense that one can be righteous while the other can be sinful, Jesus Christ was without sin and had an earthly body like ours. There is not, nor can there be, anything sinful about our flesh body itself; sin is the choice to abuse and wrongly pursue the natural, God-given desires of the flesh. When we do this, we use our body as an instrument of sin.
1Jo 1:10 KJV - If we say that we have not sinned, we make him a liar, and his word is not in us.
We cannot say we have not sinned when we have. We cannot say we have never sinned, because all have, at some point, sinned and come short of the glory of God (Rom. 3:23-26). The difference between the believing and the non-believing, the sinner and the saint, the workers of light versus the worker of darkness, is that for the Christian sin should be a thing of the past, a practice we denied when we stepped through the door of the cross into salvation. We have crucified the old man and his deeds (Rom. 6:6), and are alive anew in Christ, to dwell in the light, and should we sin thereafter, we need not lie and claim no foul, but own up to it and repent, then we are restored to the glorious light of the presence of God.
Sources and Tools: blueletterbible.com, Strong's Exhaustive Concordance, Rose Book of Bible Charts, Maps, and Timelines, Authorized KJV Bible