A difficult passage to conclude this letter. It is perhaps no wonder that one commentator suggests that it is unclear and that there is little agreement as to what it means, or even why the last sentence is there at all.
Yet certainty is there in the three-fold statement of knowledge. We know that the Christian is freed from the power of sin; that the Christian lives in a world where there is sin; and that the Christian is given understanding and discernment. Hence a Christian is able to differentiate between a life which is dominated by sin and by idols and a life lived in the truth of God as exemplified in Jesus Christ.
That of course is easy to say, but a lot less easy for the Christian to acknowledge and to put into practice. However, William Barclay commenting on this passage in the Daily Study Bible  suggests that “we live in a civilisation permeated by Christian principles … [where people] accept the ideals of chastity, mercy, service, love.” Even if that were true when Barclay was writing, I doubt if many people would be so sure that it reflected 2019 Britain.
Recent events, especially as portrayed in the media, suggest that Britons might be more likely to be unfaithful, ruthless, selfish and self-centred. This might be exemplified in the bitter debate regarding Brexit, unresolved as I write. Little charity has been displayed between the protagonists, although, mercifully, demonstrations and events have been relatively peaceful. Entrenched positions on all sides have been raised to the status of idols.
John’s charge to us as Christians is to use the freedom given to us in Christ to strive to keep ourselves and the world from idolatry. Have we so striven? Or have we just acquiesced?
 Daily Study Bible, William Barclay, The Letters of John and Jude, Revised Edition 1976, Westminster Press