But 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.
And the blood of Jesus Christ his Son cleanseth us from all sin - See the sentiment here expressed fully explained in the notes at Hebrews 9:14. When it is said that his blood cleanses us from all sin, the expression must mean one of two things - either that it is through that blood that all past sin is forgiven, or that that blood will ultimately purify us from all transgression, and make us perfectly holy. The general meaning is plain, that in regard to any and every sin of which we may be conscious, there is efficacy in that blood to remove it, and to make us wholly pure. There is no stain made by sin so deep that the blood of Christ cannot take it entirely away from the soul. The connection here, or the reason why this is introduced here, seems to be this: The apostle is stating the substance of the message which he had received, 1 John 1:5. The first or leading part of it was, that God is light, and in him is no darkness, and that his religion requires that all his friends should resemble him by their walking in the light. Another, and a material part of the same message was, that provision was made in his religion for cleansing the soul from sin, and making it like God. No system of religion intended for man could be adapted to his condition which did not contain this provision, and this did contain it in the most full and ample manner. Of course, however, it is meant that that blood cleanses from all sin only on the conditions on which its efficacy can be made available to man - by repentance for the past, and by a cordial reception of the Saviour through faith.