We know we know God because we keep his commands (2:3). The phrase “by this” are missing in the NIV perhaps because it is a transition between the previous point (claiming to be without sin) and the next point (knowing we know God).
Does “him” refer to God or Jesus? Father or son? Likely this is God the Father, although 2:2 does describe Jesus as our advocate and atoning sacrifice. Perhaps this is intentionally ambiguous: in the Gospel of John knowing Father is to know the Son, and to know the son is to know the Father.
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.
What does John mean by “keeping his commandments”? Is this the Law? Is this the Law of Christ? The command of Christ might be understood as the two great commandments, love the Lord your God and love your neighbor as yourself. But even in that context, those are summaries of the whole law. The noun ἐντολή is in the plural here, although John’s Gospel uses both the singular and the plural for Jesus’s commandment to love. John will use this noun seventeen times in the letter.
Are “his commandments” the Ten Commandments? The whole Jewish Law? There is “no hint elsewhere in the letter that the author is concerned about obedience to the Mosaic Law” (Kruse, The Letters of John, 78). Robert Yarbrough points out the word ἐντολή is not used in the LXX for the Ten Commandments, so it is not the case the “commandments” in 1 John 2 are the Decalogue (1-3 John, 82).
Knowing something implies responsibility for that knowledge. If you know what is right and good you are obligated to do what is right and good. Following C. H. Dodd, Jobes thinks this knowledge was promised as part of the new covenant (1, 2, & 3 John, 81). Jeremiah 31:34 says that when God makes the new covenant, the Law will be written on the heart so that God’s people will be able to keep it. In this case we know God and this implies our responsibility to keep his commands. But what commands? “When he uses the singular form, it always refers explicitly to Christ’s command that his followers should love one another.
Knowing God and being in fellowship with him is expressed in how we live. It is not possessing secret knowledge only revealed to the few (Gnosticism and the mystery religions), nor is it expressed religious rituals and rites (pagan sacrifices designed to placate the gods). If we know God then we know how he wants us to live. And if we know how God wants us to live, then we ought to live that way.
In the previous section of the letter, John dealt with the claim the opponents are without sin, now he is dealing with the claim they actually know God. To return to the original question, “how do we know we (really) know God?” For John, one clear indication is a person knows what God has commanded and is trying to respond to those commands. But this is not a long list of “thou shalt not” commands, John summarizes God’s demands a “love one another.”
What are some ways we can know we really know God by demonstrating love for one another? Who is included in this “one another”? Are there any limits in 1 John on who the Christian ought to love?