I used to think that walking in light means not sinning. 1 John 1:7 says, if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin. Verse 8 says, if we say that we have no sin, we deceive ourselves, and the truth is not in us. If he was talking about sinning from verse 7 then there was no need for him to say that if we say we have not sinned, we deceive ourselves immediately. If he meant that walking in light means not sinning then verse 8 would be contradicting which is impossible in its contextual meaning,
Chapter 2 of John however explains what it means to walk in light. 1 John 2:10 says: Whoever loves his brother abides in the light, and in him there is no cause for stumbling. But you can make mistakes while walking in love. I remember doing something by mistake to a friend. I was not able to pray. That fellowship was impossible because of the heaviness of heart until i went to my friend and confessed. Walking in love does not actually eliminate errors that we make. But if you chose love then you will have fellowship with God. Jesus taught a lesson about giving a gift at the alter. If someone remembers that he has something against someone, he must make things right with friend then my then the gift will be accepted. When we walk in love, we are walking in light. Since our father is light then we will be able to have fellowship with him.