The next claim from the opponents is that they not only claim to know God (but they do not keep his commands), they are claim to abide in him but they are not walking like Jesus walked.
In John’s Gospel, “abiding” is an important theme. The word can refer to remaining or staying (so the disciples who follow Jesus in 1:39 remain with Jesus, or Jesus remained with the Samaritans for two days, 4:40). The word has the sense of continuing to live in a particular state or place, even a permanent state (LXX Psalm 9:8, the Lord’s throne abides forever). In John 6:56 the one who eats the flesh of Jesus and drinks his blood “abides in Jesus.” In 15:4, the believer is like a branch which remains a part of the vine. In John 5:38, the one who does not believe in Jesus or the one who sent him does not abide in the word if God (and it does not abide in them).
The one who is abiding in Jesus ought to walk just as Jesus walked. In verses 7-8 John clarifies this is nothing new, but the same thing he has always taught them. It may be the case the opponents are saying John is teaching something new and different, so here he says this is the same thing he has always taught.

John can claim the opponents are not abiding in Christ because they are hating their brothers and sisters. This has significant application to modern church relationship, how do we relate to people who have left our church, and perhaps talked badly against us, or said things about the church which were not true? How do we relate to another denomination? Do we express “hate” toward them because they are not us? As Paul says in Philippians 1, if the gospel is preached praise God!
This person is not in the light at all, but they remain in the darkness. This is a self-deception, they have blinded their own eyes. The one who cannot see cannot know where they are going, not only dangerous but unlikely to reach the right goal.
The person who loves their brother remains in the light and there is “no cause for stumbling,” a common metaphor for sin. The verb “to stumble” is used fifteen times in the New Testament, “always with the meaning of causing some sort of harm to a person” (Kruse, 86). In this case, it has the sense of causing someone to sin. If you cause somebody to sin, then they will be under judgment to their harm.
If you are living a well-lit place, you see things that might make you stumble and you avoid them. Living in the dark is dangerous since you may stumble and injure yourself since you cannot see the danger right in front of you.
Imagine telling somebody that it is perfectly acceptable and healthy to eat a 6000 calorie a day diet. Objectively, this is very dangerous and if someone tries to eat that much every day they will become extremely fat and sick and die. The teaching is false, and it leads people to death (quite literally in this case). This new diet might actually be very popular, since you can eat whatever you want to do as much as you wanted to. However being popular does not make it right.
It is likely the opponents are teaching something that is very attractive: What we do it doesn’t count as sin or that God is not going to judge some behavior like as if it is sin. This is would be a very popular teaching, but it is also false and very dangerous.
What are some examples from modern church experience of “hating a brother or sister”? How does this animosity affect a community of believers? Is John right that one cannot hate a brother and really abide in Christ?
By “the love of God” John means the love a believer has for God, so the love for God is perfect when we obey Jesus‘s command to love one another. The ESV translates the verb τελειόω as “perfected.” This may imply (in English) “made perfect.” The verb, however, has the sense of reaching the intended goal (Jobes, 1, 2, & 3 John, 86) or even reach a state of maturity. John is talking about our transformation as a child of God by means of obeying his commands, moving from immaturity to maturity.
Knowledge in the Bible is more than just factual knowledge (that we are aware of the fact that God exists). In the Old Testament knowledge often has a sense of intimacy, so that it can be used as a metaphor for sexual intimacy (“Adam knew his wife”). Since John introduced the idea of fellowship in the introduction to the letter (1:3) and dealt with those who claim fellowship but continue to sin, it is important to understand knowledge in this context as intimate personal knowledge that comes from being in fellowship with God.
But we will sin, so Jesus is our Advocate (2:1b). The world John uses here (παράκλητος) is the same used in John 14:16 to refer to the Holy Spirit. The verb can have the sense of pleading a case, as in 2 Corinthians 5:20, Paul implores his readers to be reconciled with God. The word is sometimes translated as “comforter,” someone who encourages or comforts someone in a difficult situation.
But this analogy does not do justice to the problem because John uses very strong language to describe this person: they are a liar and God’s word is not them. That God’s word is not in them may refer to John’s own eyewitness report of the Gospel and the revelation from God he has given to his churches. This type of person is self-deluded and never understood what John had taught them in the first place. Perhaps they read passages like John 3:20-21 and misinterpreted it, thinking John meant if they were in the light whatever they did no longer counted as sin.