If we claim to have fellowship with him yet walk in the darkness, we lie and do not live by the truth. 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, purifies us from all sin. (NIV)
If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. (KJV)
If we say that we have fellowship with Him and yet walk in the darkness, we lie and do not practice the truth; but if we walk in the Light as He Himself is in the Light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin. (NASB)
1 John 1:6-7
Commentary
If we say: The first false claim is to have fellowship with God while failing to reflect His moral character. Walk refers to a way of life or daily practice. To walk in darkness means to live contrary to the moral character of God, to live a sinful life. To claim fellowship with God without living a moral life or practicing the truth is to live a lie, since God cannot compromise His holiness to accommodate sin.
Radmacher, E. D., Allen, R. B., & House, H. W. (1999). Nelson's new illustrated Bible commentary (1 Jn 1:6). Nashville: T. Nelson Publishers.
Light
The New Testament presents Jesus as the personification of light or divine illumination: "I am the light of the world" (John 8:12). Jesus plainly stated that those who rejected this divine light would bring judgment upon themselves (John 3:19-21). Jesus and the New Testament writers extended the figure of light to include faithful Christian witnesses, who were called "children of light" (Eph. 5:8).
Youngblood, R. F., Bruce, F. F., Harrison, R. K., & Thomas Nelson Publishers. (1995). Nelson's new illustrated Bible dictionary. Rev. ed. of: Nelson's illustrated Bible dictionary.; Includes index. Nashville: T. Nelson.