The core doctrines of the gospel are simply put to the reader by John in terms anyone even child can understand. From these core points we can expand the doctrine outward. The over arching truth is that any notion contrary to these points is either a lie by us or else a lie by Him; there is no middle ground. Light, fellowship with the Light, fellowship with each other, sin, confession of sin, forgiveness of sin and the cleansing of sin from unrighteousness are all a one way or no way proposition. Who shall be the liar here; man or Christ?
To abide in Him is to love and to walk as He walked. This does not come naturally. The special anointing that we receive as repentant believers teaches us of all new and necessary spiritual things. We are taught by abiding in Him, abiding is our school. We love as He, we learn. We walk as He, we learn. How then can we love unless we abide in Him? His love is not just any love, His walk not like any either. This love and walk we are unfamiliar with even to the point of being at enmity with it. It is only by His unction that we are able to do it. Some then by not abiding have come out from us and have become our opposite, the antichrists.
There is so much said here that entire books can be (and have been) written. For the moment it should be enough to consider that these things are all expansions of the core doctrine that John presented in kjv@1John:1 , namely that God is light and in Him is no darkness. If we are in God, so too there can be no darkness in us. Knowing the 'how' this is possible is knowing the 'what' Jesus accomplished in His death and resurrection and the 'who' He is. The working of this knowledge produces unfeigned love in us for the brethren, which is the proof positive of possessing this knowledge. It can be produced in no other way.
Cain is purposely used as the counter example. This key information steers the understanding of this passage a direction it otherwise would not go. The context becomes the inseparable fusion of love and righteousness, it's perceived source/manner and the resultant actions occurring from. Two men make the same effort to worship the living God, the means of which produce two opposite ends. The same can be said of two men that worship, one finding the ability to love unfeigned, the other finding the ability to hate and inflict judgment.
If I were to ask nearly any non-believer 'what is God?' the near unanimous reply would be "God is love". If I were to ask then 'what is love?' I would receive a multitude of varying replies mostly having something to do with tolerance for their sins. The question then to ask is 'doesn't that mean that love is whatever one wants/needs it to be?' or better 'that God is whomever we want/need Him to be?'. What kind of god can we ourselves make up? Is your wife whomever you wish her to be? Is your son? Is there any other working relationship that you know of that is determined by what you wish it to be? Are we not individual? Do we not have structure and backbone, interests and opinions and needs of our own that you yourself have to accept navigate and familiarize your self with? Isn't that the beauty of relationships? Why should it be any different with God? We love God because he first loved us. It was not our minute and varying personal perceptions of God with which He loved us, it was His gigantic eternal design for present and future, a love that would redeem us from our sins and set us aright into eternity. It was not our selfish 'I need you to be this' love or 'do this now for me or else' love or 'if you even exist' love for His love came before our love. What then is love? God is love? What is God? Creator and perfecter and possessor of our souls in whom no darkness dwells and in that He is absolute love.
I find the rhythm in the end odd. Such a metered and descriptive examination of godly love throughout the passage, to end in such an abrupt change of cadence 'keep yourself from idols' (out!). Is the transition from love to idols as so abrupt however? Are they not essentially the same thing? All that he has spoken of love and heavenly wittiness and sinlessness and divine providence, can they not be wrapped up in the few words of keeping yourself from idols? Where then do our idols exists?
The idols spoken of here can be as simple as a Jesus other than the one testified of by the Father. A Jesus that isn't God made flesh. A Jesus who is not His only begotten Son. A Jesus who is not His beloved. A Jesus who is one of many ways acceptable to the Father. Any other Jesus makes this Jesus a liar. Try this translation: "keep yourselves from the Jesus that makes this Jesus a liar".