If we claim to have fellowship with Him
yet walk in the darkness,
and do not live by the truth.
First, I want to point out that it is possible to claim fellowship with God, yet have no idea what that is all about. It is possible to say with your mouth one thing, but in your life, completely deny those words.
The truth that is wrapped up in this verse, and so many others like it, is a truth that has been overlooked in so many of the churches today. We are so caught up in God's love and grace, that are real and true... that we forget that we also must be real and true.
Our walk absolutely must be a representation of the work of God in us. If it is not, then the reality is that God is not working in us. But Philippians 1:6 says, "Be confident of this very thing, that He who began a good work in you will perform it until the day of Christ." The work of salvation is Christ's work. He began it, He is going to perform it, and He will complete it.
Or what about Ephesians 2:8-10, which says, "For it is by grace you have been saved, through faith; and this not from yourselves, it is the gift of God; not by works, so that no one can boast." (We love that first part don't we? See, we say, no works required for salvation! ... But keep reading. The passage doesn't stop there.) "For we are God's workmanship, created in Christ Jesus to do good works, which God prepared in advance for us to do."
Can you see it? Works is the natural result of true salvation. Because that is what salvation is! It is a saving from one walk (or way) of life, into a different walk - a walk in the light!