1 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; 2 (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;) 3 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. 4 And these things write we unto you, that your joy may be full.
In these first few verses the Apostle John is simply stating who he is. That he walked with Jesus , was taught by Jesus and that he is getting ready to share that which he learned.
5 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. 6 If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: 7 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.
This is the message he is sharing. God is light and in him is no darkness. We cannot be full of darkness and have fellowship with God. If we believe however that Jesus was the savior and has quickened us out of darkness into the light we will have fellowship with one another. He is also establishing a comparison between darkness (sinner) and light (child of God).
8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
This verse is often misunderstood. What John is saying is that we were all born in darkness with the nature of sin. This is the state of man prior to believing and accepting Jesus.
9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
This verse is the solution to verse 8. When we acknowledge that we are sinners, we repent, accepting the finished work of Jesus crucified. We are forgiven and made righteous by grace through faith in Christ Jesus. Having done this we know that we are born again, no longer in darkness (a sinner), but in the light (son of God).
10 If we say that we have not sinned, we make him a liar, and his word is not in us.
God had concluded the whole world was in sin when he gave us an image of righteousness through the law and as sinners (darkness) we could not keep it. If we say that we are righteous without having believed in the only begotten son of God and the work of the cross we are calling God a liar because we have not received the testimony of God concerning Jesus who would in fact bring salvation (light) to a world bound in darkness (sin). Reference John 3:16-19.