If we claim we have no sin, we are only fooling ourselves and not living in the truth.
Insight
Here, John was attacking the second claim of the false teachers: that people had no natural tendency toward sin, that they “[had] no sin,'' and that they were then incapable of sinning. This idea is at best self-deception and at worst a bald-faced lie. The false teachers refused to take sin seriously. They wanted to be considered Christians, but they saw no need to confess and repent. The death of Christ did not mean much to them because they didn't think they needed it. Instead of repenting and being purified by Christ's blood, they were encouraging sin among believers.
Challenge
In this life we are always capable of sinning, so we should never let down our guard.1 Samuel 7:12 Hitherto hath the Lord helped us.The word "hitherto" seems like a hand pointing in the direction of the past. Twenty years or seventy, and yet, "hitherto the Lord hath helped!" Through poverty, through wealth, through sickness, through health, at home, abroad, on the land, on the sea, in honor, in dishonor, in perplexity, in joy, in trial, in triumph, in prayer, in temptation, "hitherto hath the Lord helped us!" We delight to look down a long avenue of trees. It is delightful to gaze from end to end of the long vista, a sort of verdant temple, with its branching pillars and its arches of leaves; even so look down the long aisles of your years, at the green boughs of mercy overhead, and the strong pillars of lovingkindness and faithfulness which bear up your joys. Are there no birds in yonder branches singing? Surely there must be many, and they all sing of mercy received "hitherto." But the word also points forward. For when a man gets up to a certain mark and writes "hitherto," he is not yet at the end, there is still a distance to be traversed. More trials, more joys; more temptations, more triumphs; more prayers, more answers; more toils, more strength; more fights, more victories; and then come sickness, old age, disease, death. Is it over now? No! there is more yet-awakening in Jesus' likeness, thrones, harps, songs, psalms, white raiment, the face of Jesus, the society of saints, the glory of God, the fulness of eternity, the infinity of bliss. O be of good courage, believer, and with grateful confidence raise thy "Ebenezer," for-- He who hath helped thee hitherto Will help thee all thy journey through. When read in heaven's light how glorious and marvellous a prospect will thy "hitherto" unfold to thy grateful eye! | 
