1 John 1:6

If we claim to have fellowship with him yet walk in the darkness, we lie and do not live by the truth.

(v6, NIV)

As John mentioned in the last verse, God is light. Not only is God light, but there is no darkness in him. Since God is light, it would make sense that he would take no part in anything having to do with darkness; this includes those who walk in darkness. John asserts very plainly that you cannot walk in darkness and have fellowship with him. It just doesn't make sense for those walking in darkness to have fellowship with God. In fact, John is addressing certain people in particular who claim to have fellowship with God, yet walk in darkness. There are people like that, people who say that God is their buddy and that they have fellowship with him, yet they walk in darkness. Maybe these people who claim to have fellowship with God live a certain way that is not of the light, but of the darkness. Regardless, those people are liars in their claim to have fellowship with God. The way and path of their lives is not of the truth.

What's interesting is that John continues to use the word "we". He is including himself among with everyone else. It would be silly for him to exclude the possibility of him being one of these liars. It would be prideful to think he was immune to this sort of thing, which he may be deceived into thinking that he was really in fellowship with God yet walking in darkness. He includes everyone by saying "we", addressing those who have seen, heard and touched the living Christ. In addition, these people hear, testify and proclaim to the word of life. However, if for some reason "we" say we have fellowship with God yet walk in darkness, we are liars.