First John is evidently written to a congregation (or perhaps multiple congregations) into which some false teachers had entered, and which were being disturbed by the things they were hearing from these. So John says: I write these things to you concerning those who would deceive you (2.26). And among the various problems and doubts raised by these false teachers, it seems also that persons in John's congregations were worried whether they really had come to know God in Jesus Christ through the apostolic preaching.
Obviously this is a problem with which many (if not all) of us can relate: we wonder to ourselves in moments of doubt and darkness whether we really have come to know God after all, if we are not deceiving ourselves in this whole Christian life we are trying to live. John has a solution for this problem:
Little children, let us love, not in word or speech, but in truth and action. And by this we will know that we are from the truth, and will reassure our hearts before him whenever our hearts condemn us (3.18-20).
For John, the evidence that you come into contact with the true God is that you love; for he says a little later on, Whoever does not love does not know God, for God is love (4.8). True knowledge of the truth, true religious life, has an intrinsically ethical aspect for John: to be from the truth means to have a certain orientation towards a particular kind of life, a demonstration of the intervention
But of course, this might seem far away from many of us. Who can say that their life is characterized by the kind of perfect love in truth and action that John calls us for? His standards are high: we ought to lay down our lives for one another (3.16). Can any of us claim that we are willing to do something like that for even the most annoying and most troublesome members of our congregations?
John's standards are high because he knows love as it was revealed in Jesus, but neither is he an unrealistic idealist. He knows from the start we that we have sin and we are not perfect: If we say we have no sin, we deceive ourselves (1.8). It seems to me that John would include even those persons who want to love but do not have it at the moment as having had some kind of genuine contact with God.
After all, those who are utterly removed from God are without love, but they also don't want to love, either: they don't see the point in it, they can't imagine such a thing, they don't feel any attraction whatsoever towards a life of love. But if you at least want to love, if you see in a life of love something beautiful and worth aspiring to, you cannot be utterly removed from God; you have to had some kind of contact with God's grace -- and it remains for you to cooperate with this grace and to develop into the loving person you know you are being called to be.