1 John 1:6 If we say that we have fellowship with Him, and walk in darkness, we lie and do not practice the truth.
If I say that I am walking with God, talking back and forth with Him, presenting myself before others as this christian, while actively sinning, I lie and am not practicing truth. Its really this simple. Its not the commiting of sin, for we do this everyday, often unaware at first. Its the lifestyle and practice of sin, the natural default unto it. I must have an honest assessment of my life before Gods word and give Him my heart daily. A christian will sin but its inconsistant for us to think we are christians while practicing & walking in sin. LORD examine my heart today, wash me and keep me from the lures of sin.