1 John 2:9

Anyone who claims to be in the light but hates his brother is still in the darkness.

(v9, NIV)

This time the contrast is between loving your brother and hating your brother. If you don't have any brothers, you're not necessarily immune to this. The "brother" being referred to here is your brother in Christ. These are other believers that John is referring to. So, there are two options, you either hate your brother or you love your brother. These two options are together with the claim to be in the light.

This is the message we have heard from him and declare to you: God is light; in him there is no darkness at all.

(1 John 1:5, NIV)

If you recall from chapter 1 of 1 John, God is light and in him is no darkness. God is light and the claim is to be in him. Do you claim to be one of his? Do you think that he knows you and are in fellowship with him? If so, you'd better check to see whether you hate your brother or love your brother. If you hate your brother, then you are in darkness. Your claim to be in the light is contrasted with the reality that you are in the darkness. These are two complete opposite states. You are either in the light or in the darkness. In addition, there is no darkness in God. Your claim is to be in the light, to be in God. However, there is no darkness in God. Therefore, if you truly are in the darkness then you have no part with God.

So, why is it that loving your brother or hating your brother determines whether you are in the light or not?

We love because he first loved us. If anyone says, "I love God," yet hates his brother, he is a liar. For anyone who does not love his brother, whom he has seen, cannot love God, whom he has not seen. And he has given us this command: Whoever loves God must also love his brother.

(1 John 4:19-21, NIV)

Later in chapter 4 of 1 John, a similar contrast is made between those love their brother and do not love their brother. These people say they love God, but this cannot be true if they don't love their brother. It doesn't make sense for those who claim to love God yet do not love their brother. These two do not match up and both cannot be true. Therefore, your claim to love God is a lie.

However, coming back to chapter 2 of 1 John, there is a more prominent reason why this makes sense. John has just finished talking about this commandment which is both old and new. The commandment is to love one another, following in Christ's example. It's not explicitly stated here, but we can gather enough information from what John says in his letters and in his gospel account.

What's interesting is how all of what John is writing all connects and is linked to truth and light and love. The truth in the command to love one another is seen in Jesus and in those who follow it. Those who obey this commandment are not lying when they say that they know him. Therefore, the truth is in him. In addition, the truth is not in you if you claim to be without sin. This also applies if you claim to have fellowship with God yet walk in darkness. Truth and light go together. In the same way, lying and darkness go together. You cannot be in the light yet speak lies. Similarly, you cannot be in the darkness yet speak the truth. These are the contrasts of the things John has been talking about.

The command John was just talking about is the command to love one another. If you are not following that command, you do not love your brother and are instead hate your brother. Those who disobey Jesus' commands cannot claim to know him without lying. By doing so you are walking in darkness. Therefore, those who hate their brother are walking in the darkness; these people cannot claim to be walking in the light.

Added: Here, John addresses those who say they are in the light. If these people hate their brother, they are in fact still in the darkness. Their claim to be in the light is wrong. They claim to be in the light, but they are in fact still in the darkness. The word "still" implies that those who are in the light used to be in the darkness. There is a progression from darkness to light, but these who hate their brothers have not made this progression and are still in the darkness. In addition, "still" is referring to now. So, currently, they are in the darkness. However, that doesn't mean that they will remain in the darkness. There is the possibility that though they are currently still in the darkness, they may not remain in the darkness.