Why does the apostle consider that the denial of man of his sinfulness is a false witness on Jesus? Of course, one could say that it is, first of all, about the sense of the earthly ministry of the Savior, Who said directly that the world lies in evil and that He came to deliver him it from this evil. Hardly in such a context our assertions of our own sinlessness will sound even a little convincing; they will become rather the witness of our thoughtlessness or arrogance.
Maybe and something else: John could answer in his message those of his coreligionists, who were (perhaps, quite sincerely) persuaded that having entrusted their fate to Christ, they had freed themselves just like that from the power of this sin, which prevented them from living formerly a righteous life. Knowing that sin and the Kingdom are incompatible and being assured that the Kingdom became an integral part of their life, they completely calmed down for their own sinfulness: it seemed to them that they are now not under the power of sin by definition.
The apostle, as seen, dispels their errors on his own account. But the main thing is all the same, probably, somewhere else: the denial by man of his own sinfulness makes very absurd the coming of the Savior into the world. Isaiah of Babylon already said that the Messiah is not going to suffer for His own sins, but for the sins of the people, to whom He will come. And the cleansing of the people from sin will become one of the essential tasks being before Him.
Jesus Himself spoke in particular about the salvation of the sinners from their sin, without cleansing of which one can not enter the Kingdom. But here is, it turns out now that were found in the Church people questioning this mission of the Savior. It is not surprising that John gives them a severe rebuke: for, not wishing to accept the proposed salvation, they not only did not go to Christ, but also misled the others on the path. John understood perfectly that it is about very important things, not to be treated so lightly.
