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. But if we walk in the light - If, having received the principle of holiness from him, we live a holy and righteous life, deriving continual light, power, and life from him, then we have fellowship one with another; that is, we have communion with God, and God condescends to hold communion with us. This appears to be the intention of the apostle; and so he was understood by some versions and MSS., which, instead of μετ' αλληλων, with each other, have μετ' αυτον, with him. Those who are deeply experienced in Divine things converse with God, and God with them. What John says is no figure; God and a holy heart are in continual correspondence.
The blood of Jesus Christ - The meritorious efficacy of his passion and death has purged our consciences from dead works, and cleanseth us, καθαριζει ἡμας, continues to cleanse us, i.e., to keep clean what it has made clean, (for it requires the same merit and energy to preserve holiness in the soul of man, as to produce it), or, as several MSS. and some versions read, καθαριει and καθαρισει, will cleanse; speaking of those who are already justified, and are expecting full redemption in his blood.
And being cleansed from all sin is what every believer should look for, what he has a right to expect, and what he must have in this life, in order to be prepared to meet his God. Christ is not a partial Savior, he saves to the uttermost, and he cleanses from All sin.