It would seem in John’s world, much like our own, there were people who believed in God but carried a sin-denying theology. In his countering of these claims he calls these people liars (v6), lying to themselves (v8) and in this third response he says they lie to God.
“If we claim we have not sinned, we make him out to be a liar and his word is not in us.” (1 John 1 v 10)
Today in churches throughout the world visitors will gather and our prayer is that they come to know Jesus who gave His life for them. “…his word …” which John means either the Scriptures or Jesus Himself can only occupy a place in the lives of those who know they have sinned against God. To have a Saviour you need to be a sinner.
No one is perfect. We cannot fix sin ourselves.
But we can ignore it, rationalise it and deny the glory of God that sin falls well short of (Romans 3:23).
John is clear in the opening of his letter. To truly know God is to have first known sin.
Missions Paul 