By receiving the message that the Apostles proclaim, we join them in fellowship as they are in fellowship with God.
1 John 1:5-10 This is the message we have heard from him and proclaim to you, that God is light, and in him is no darkness at all. (6) If we say we have fellowship with him while we walk in darkness, we lie and do not practice the truth. (7) 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 cleanses us from all sin. (8) If we say we have no sin, we deceive ourselves, and the truth is not in us. (9) If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. (10) If we say we have not sinned, we make him a liar, and his word is not in us.
John continues from the previous passage where he encouraged us to join him and the other Apostles as they are joined in fellowship with God. He gives a summary of the message that they have heard and now proclaim to us. God is Light and in Him there is no darkness (v5).
He then expands upon the imagery of light and darkness and uses it as a means to start describing what it means to have fellowship with God. It means having fellowship with one another(v7). It means having our sins cleansed by the blood of Jesus (v7). It means acknowledging our sin (v8). It means confessing our sins to the one who can cleanse us from all unrighteousness (v9).
To have fellowship with God, who is the Light, is to first realize that we are sinful and lost in the darkness. If we continue in the darkness (v6) and say we have no sin (v8 and v10), we are liars and deceive ourselves. And the worst part is that we remain in the darkness. Truth is acknowledging that we are sinful, unrighteous and lost in the darkness. Truth is acknowledging our state before God who cleanses us from all unrighteousness. Truth is having fellowship with God and walking in the light.