"..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 cleanses us from all sin".
This Word is coined from the Greek word 'katharizo' which means - 'to purge, to purify, to make clean'. It is the same word from which we get the medical term 'catheter'. A catheter is a medical instrument (tube) that is inserted into the body of a diseased person for the purpose of continuous drainage, purification and removal of impurities from the body. This tube continually 'cleanses' the body making it free of all bad substances.
In the same way, John is affirming to us that the Blood of Jesus is a 'catheter', constantly purifying the believer of all sin, impurities and wrong doing. It (Jesus' blood) is the singular substance required for the continual cleansing of the believer.
Please note that this is an action that does not depend on you or what you do. That is, the cleansing is not triggered by any external action on your part. Just as the medical catheter works constantly (irrespective of the patient's condition, as long as it is inserted into him) to remove bodily impurities, the blood of Jesus works in the same way to always ensure that in the sight of God, you are always presented spotless, blameless, sinless and guiltless.
God's own catheter is at work to ensure that no matter the circumstance, there is NO CONDEMNATION to you who is in Christ Jesus. Therefore rejoice believer, you are purified by Jesus' blood. No need to go about feeling helpless or ashamed or worse, powerless. No use seeking an external vindication of your acceptance before God, or your worth to him.
The 'catheter' has made you clean. His blood cleanses you from all sin!