1:7 fellowship one with another. That is, we are in fellowship with the Lord, and therefore also with other believers who are in fellowship with Him. Since there is no darkness in God, if we truly walk in His light, there can be no reason for any error, sin, or ignorance of His will on our part.
1:7 all sin. All sin, whether known or unknown, is cleansed by His blood, as we walk in fellowship with Him.