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 cleanseth us. Literally this could read “keeps on cleansing us from all sin.” The blood of Christ was not like that of other men, for it was “the precious blood…without blemish and without spot” (I Peter 1:19), uncontaminated either by inherited genetic mutations or inherent sin. Somehow, after it was all poured out at the foot of the cross (John 19:34) in atonement for our sins (Hebrews 9:22-28), “by His own blood [Christ] entered in once into the holy place, having obtained eternal redemption for us” (Hebrews 9:12).

1:7 all sin. All sin, whether known or unknown, is cleansed by His blood, as we walk in fellowship with Him.