If we say that we have fellowship with him and walk in the darkness, we lie, and don't tell the truth.

 The Greek for to cleanse is katharizein ( G2511) 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." 