1 John 2.3-6 (ESV), And by this we know that we have come to know him, if we keep his commandments. Whoever says “I know him” but does not keep his commandments is a liar, and the truth is not in him, but whoever keeps his word, in him truly the love of God is perfected. By this we may know that we are in him: whoever says he abides in him ought to walk in the same way in which he walked.
By this we know that we have come to know him – “we know” (present tense) “we have come to know” (perfect tense). We know it right now but that’s because we knew in the past and the effects abide with us. We know “him,” that is, Jesus. This is a once-for all obtained and continued in knowledge. There are abiding effects that come with this knowledge.
If we keep his (Jesus’) commandments – God’s commandments are a revelation of light and darkness (v.6, 7). By obeying them, we know God and continue to know Him. You won’t keep them perfectly; you ought to try. Commandment keeping assures of something: we know God. The guy in 1.6 does not know God (thinks God does not care about sin); he’s not keeping the commandments. The one who knows God keeps the commandments of God, i.e. walk in the light (which God has revealed).
“I know him” – should be the same as above: I have come to know him (perfect tense Gk.).
Does not keep his commandments – seems to acknowledge the standard but does not live according to what God has revealed through Christ. We must acknowledge the standard and live accordingly.
[he] is a liar – this is not just a boo-boo; this is serious stuff. And from the “apostle of love”! Is there love in calling someone a liar? There is with God; he knows the reality and gravity of the situation. To call darkness “darkness” is a kind thing to do – like putting a skull-and-cross-bones on a bottle of poison or a lighthouse on the shore, these are acts of love. This is an expression of divine love. Otherwise, we end up with an Isaiah 5.20 situation.
Keeps his (Jesus) word – obedience to the commandments
The love of God is perfected – John will go into deep detail about perfected love (cf. 4.7ff.) God has a deep, vested love, a cross full of love. We’ll see that. But here, suffice it to say that keeping the commandments of God, the word of God will bring about perfect love in us. That is, it is an accomplished fact, a reality. This is our love for God; knowing God necessitates keeping God’s word (commandments) and keeping His word involves loving Him. Overarching this is being “in him” which is fellowship language – with Him, His Son.
We may know that we are in him – this is fellowship language. By this – what? Keeping his commandments. This confronts and conflicts with antinomianism.
There is obligation that goes along with our profession. You’re in Christ (conversion), how do you remain in Him?
Whoever says he abides in him – here is profession: “I am in Him.” How many people claim this? They claim to be in Christ, in God. John is going to confront those, not only in his day, but even in our day who claim to be something they are not: Christian. For us today, those Christians who profess to follow Christ but live like the world, that kind of conduct is just heretical as any of these first century threats. “There is something frightful in the fact that the most dangerous thing of all, playing at Christianity, is never included in the list of heresies or schisms.” –Soren Kierkegaard, Danish philosopher and theologian.
Ought to – here is our obligation. “Ought to”: really feel the weight here; you owe it to God to live like Christ.
walk in the same way in which he (Jesus) walked – How did Jesus walk? Exactly as God commanded, assigned. He walked in love, righteousness, mercy, peace. If would say we abide in him, we have the moral obligation to follow the perfect model of our faith and walk as Jesus walked. Nothing less than perfection should be our aim in living for God.