If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: 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. 1 John 1:6-7
He who really knows the truth is daily more and more cleansed from sin by the blood of Jesus. The verse is correct enough here but it can very easily be misunderstood. It runs: “The blood of Jesus his Son cleanses us from all sin.” That can be read as a statement of a general principle. But it is also a statement of what ought to be happening in the individual life.
The meaning is that all the time, day by day, constantly and consistently, the blood of Jesus Christ ought to be carrying out a cleansing process in the life of the individual Christian. The Greek for to cleanse is katharizein which was originally a ritual word, describing the ceremonies and washings and so on which qualified a man to approach his gods.
But the word, as religion developed, came to have a moral sense; and it describes the goodness which enables a man to enter into the presence of God. So what John is saying is, “If you really know what the sacrifice of Christ has done and are really experiencing its power, day by day you will be adding holiness to your life and becoming more fit to enter the presence of God.”
Here indeed is a great conception. It looks on the sacrifice of Christ as something which not only atones for past sin but equips a man in holiness day by day.
(Barclay, William, The Letters of John and Jude Revised Edition, Westminster John Knox Press, Louisville, Kentucky, 1976, pgs 30-31)