Those who walk in the light can be certain that their sins, though they were like scarlet, have been washed as white as snow (cf. Isa. 1:18). To know that the blood of Christ keeps on cleansing us from our sins is a blessed assurance, but some might think that it is too good to be true. Isn't there a condition in order for Christ's blood to keep cleansing us of sin? Indeed there is. We must "walk in the light as He is in the light" (I John 1:7). But what does that mean? How exactly does one walk in the light? Does it mean always doing what is right and never falling short? That's what some seem to think it means, but that is not correct. To "walk in the light" is not the same as "living completely without sin." First of all, living sinlessly is impossible as we already noted in I John 1:8,10. Secondly, if walking in the light meant to live sinlessly, then this verse wouldn't even make sense! Think about it: Why would John say that the blood of Christ cleanses you from all sin if you live sinlessly! That's nonsense.
So, what is John saying here? What does it mean to "walk in the light"? Perhaps the best way to understand this phrase is to keep in mind the distinction between perfection and direction. The issue is not whether we have obtained perfection, but whether we are going in the right direction.
Remember when Jesus talked about the strait and narrow way that leads to life (cf. Matt. 7:13,14)? He used this illustration to teach that we must stay on the right path. If we choose to take another road, then we fall from grace. It is possible to fall, and when one turns away from the right road and takes another one, he is in apostasy. But, just because you stumble and don't walk as well some days as others doesn't mean that you've left the correct path. Sometimes it may seem hard to keep going, but as long as we stay on the right path by striving to do our very best for the Lord and His church, then we are walking in the light and the blood of Christ keeps on cleansing us from all sin.
My wife cannot honestly say that I am perfect as a husband, but she can say that I am faithful to her. In the same way, God knows that we are not perfect Christians, but He expects us to be faithful Christians! To be faithful is to walk in the light; it's to keep going in the right direction with the Lord (as defined by the New Testament). As long as we walk in the light, we have the blessed assurance that the blood of Christ keeps on cleansing us of our sins. We should cling to this wonderful truth!