The joy of the Apostle John is made complete, as he opens 1 John with a discourse on the fellowship that exists between the believer and God the Father, and his Son, Jesus. John wants readers of this letter to be diligent and continue in the faith because the gospel message they have learned is real and true.
John asks the question in chapter 2 that has become a question multitudes have asked, "How can we be sure we belong to Jesus?" Do we wrestle with that? Do we have any assurance that we really belong to him? John answered his own question by letting us know the only real answer is this, "By obeying his commandments." John continues to elaborate, "Those who say they live in God, should live their lives as Christ did."
Now look in the following verses, 1 John 2:7-8
"Dear friends, I am not writing you a new command but an old one, which you have had since the beginning. This old command is the message you have heard. Yet I am writing you a new command; its truth is seen in him and you, because the darkness is passing and the true light is already shining. Anyone who claims to be in the light but hates his brother is still in the darkness. Whoever loves his brother lives in the light, and there is nothing in him to make him stumble.
In the writings of Paul to the Colossians, he says that God has rescued us from darkness and brought us into the kingdom of the Son he loves. That is an amazing way to describe the accomplishment of Jesus on the cross. We have such a blessing in escaping the darkness of sin and instead, living with Jesus.
But note, in the quoted passage above, we are commanded to love our brother. John says even those who claim to be in the light while hating their brother, are in reality still in darkness. Of special note is his comment about the process of the darkness passing and the true light is already shining. There may be times when we are attempting to walk in the light, and a wrong decision or a sinful habit pulls us back toward the darkness. John is showing us it is futile for us to attempt to be light, when the darkness of hatred toward each other shows its ugly head.
The theme of the letter is to remain faithful, and that theme is echoed in the life of every Christian in today's world. John wants us to know that we belong in the light, and tells us that assurance can be ours if we are obedient to God, and loving toward each other.