If we say that we have fellowship with him and walk in the darkness, we lie, and don't tell the truth.
"Do not the truth" simply suggests that the life is not consistent with the truth revealed in God’s word (John 17:17). The practice does not conform to the teaching. Vincent quotes Westcott: "Right action is true thought realized. Every fragment of right done is so much truth made visible" (315). Our actions should give substance or visibility to God’s word. We must both preach truth and practice truth. The scriptures suggest various relationships to truth, speaking of: holding the truth (Romans 1:18), obeying the truth (Romans 2:8; Galatians 3:7), walking according to the truth (Galatians 2:14; 3 John 1:4), resisting the truth (2 Timothy 3:8), and erring from the truth (James 5:19). The Bible emphasizes the importance of our lives being properly related to the truth of God’s word.