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.
We walk. (Ephesians 5:8; Ephesians 5:11-14) 'God IS (essentially, in His very nature, as "light," 1 John 1:5) in the light.' WALKING in the light, the element in which God IS, is the test of fellowship with Him. Christ, like us, walked in the light (1 John 2:6). Alford, 'Walking in the light, as He is in the light, is no mere imitation of God, but an identity in the essential element of our daily walk with the essential element of God's being.'
We have fellowship one with another - and of course with God (understood from 1 John 1:6), without having fellowship with whom there can be no Christian fellowship one with another (cf. 1 John 1:3). To believe, with John, is eternal life; to love, is blessedness; to serve, is usefulness.
And - as the result of 'walking in the light, as He is in the light.'
The blood of Jesus ... cleanseth us from all sin - daily contracted through the weakness of the flesh, and the power of Satan and the world. He is speaking, not of justification through His blood once for all, but of present sanctification ("cleanseth"), which the believer, walking in the light, and having fellowship with God and the saints, enjoys as his privilege. Compare John 13:10, 'He that has been bathed х leloumenos (G3068)] needeth not save to wash х nipsasthai (G3538)] his feet, but is clean every whit.' Compare 1 John 1:9, "cleanse us from all unrighteousness:" a further step besides 'forgiving us our sins.' Christ's blood is the mean whereby, being already justified, and in fellowship with God, we become gradually clean from all sin which would mar fellowship with God. Faith applies the cleansing, purifying blood. 'Aleph (') B C omit "Christ;" A retains it.