Key to koinonia (fellowship or communion) in the teaching of John, according to 1 John 2, is keeping the commandments (v. 3), keeping Jesus' word (v. 5), and walking in the same way that Jesus walked (v. 6). But what are the commandments? What is the word of Jesus? What is the way Jesus walked - thinking of the difference between the physical way of dusty roads in Palestine and the exemplary way of obedience to the Father, love for people, prophetic teaching, healing, etc?
John helps us in the answer to the question by writing in vv. 7-10:
'Beloved, I am writing you no new commandment, but an old commandment that you had from the beginning. The old commandment is the word that you have heard. At the same time, it is a new commandment that I am writing to you, which is true in him and in you, because the darkness is passing away and the true light is already shining. Whoever says he is in the light and hates his brother is still in darkness. Whoever loves his brother abides in the light, and in him there is no cause for stumbling. But whoever hates his brother is in the darkness and walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes.'
Well, John "helps" us if we like living with paradox: 'no new commandment ... it is a new commandment'!! Note the way in which verbs in these verses directly correspond to important verbs in 2:4-6: 'abides' (v.6, v. 10), 'walks' (v. 6, v. 11). This suggests that 2:7-11 is John's own interpretation of 2:4-6, at least in the sense of drawing out the most important 'commandment'. Or, perhaps we are closer to the mark if we say that John in vv. 7-11 provides the key examination question for whether we keep/abide/walk in the commandments/word/way of Jesus: do you love or hate your brother?
(It would be presumptuous to make a conclusion about John's understanding of the Christian's attitude to the Mosaic law on the basis of these few enigmatic verses. What we can appropriately note is that the question of loving or hating one's brother, in the context of talk of commandments, is coherent with (but not the same as) Pauline theology of the Mosaic law, 'The commandments ... are summed up in this word: "You shall love your neighbour as yourself".' (Romans 13:9); and consistent with Pauline understanding of disputes in church, 'for there must be factions among you in order that those who are genuine among you may be recognized' (1 Corinthians 11:19).)
Back to 1 John 2. 'Brother' here means a member of the body of Christ, a Christian sibling within the family of God, a fellow participant in the fellowship of the church. At the level of pure logic, fellowship exists where there is love between fellows and fellowship is broken if there is discord between fellows. At the level of actual living some 'deception' is possible: 'OK, so I can't stand X or Y, but that does not mean I cannot call myself a true Christian and member of their church.' The 'deception' is that either I do not, in the end, hate X or Y, or I am not able (or they are not able) to sustain membership of the same church ... or, one of me or X or Y is not actually a 'brother'! Honesty compels, or should compel Christians to confront their own attitudes to fellow Christians: do I love X or hate X? If the latter, am I prepared to change attitude? No idle question as John points out that 'whoever hates his brother is in the darkness' (2:11).
These thoughts directly relate to goings on in the Anglican Communion through these days. The language of commenters on the internet, for example, is the language of 'love' and 'hate' ... love for those perceived to be in agreement, hate for those perceived to be in disagreement ... and oft-times it is also language about the validity of 'brothers', questioning whether (say) a person disobeying this or that commandments is a true Christian or (say) a person upholding this or that commandment (against homosex) is a true Christian.
The troubling thing, speaking as a conservative who haunts conservative sites (though not only conservative sites), is that the language of 'love' and 'hate' appears when discussing fellow conservatives as well as when discussing liberals!
Perhaps it could be important to presume all members of the Communion are 'brothers and sisters' in Christ, to not waste energy on wondering who is and who is not, and to Spirit-fully examine ourselves as to whether we love or hate our brothers and sisters in the Communion? If we only love those we think to be of common mind with ourselves, we are not making much of a demand on ourselves! (See Matthew 5:46). If we are tempted to hate those we disagree with, we run the risk of being in the darkness despite feeling that our adherence to true doctrine ensures we are enlightened.
John's challenge to us is that doctrine is demonstrated by deeds, communion is expressed through charity more than clarity. Creedal conviction is important to John (e.g. 1 John 5:1a) but it is never separable from commitment to fellow Christians (1 John 5:1b).