In the previous section of the letter John explained the one who knows God does what God has commanded. What about people who do claim to know God but choose to not keep his commands? The immediate application is likely to those who went out from John’s church, the opponents the apostolic teaching.
The way John expresses this statement is important. The verb is in the perfect tense, implying an action in the past which has some effect in the present of the speaker. In this case, the opponents have (already) said they know God and they are still claiming this at the time John writes. Maybe this is an over-interpretation, but imagine a person who made a public confession of faith, perhaps submitted to baptism, gave their testimony in front of the congregation, etc., and the continue to make that profession of faith by continuing to attend church and give to charity, etc.
If someone claims to know God but does not keep his commands, they are choosing to not know what God has revealed about himself and his will. They are a liar because they actually did not know God in the first place.
On the other hand, the one who does obey God‘s word, God‘s love is truly made complete in him. This is not a conditional sentence, although appears to be in the NIV. Notice that commandment and word are used in parallel here. To keep God’s word is to keep his commandment (cf., 1:8 and 1:10, see Lieu, I, II, & III John, 70). The very translated “obeying” is in the present tense, so an ongoing action. But “made complete” is in the perfect tense, so the past tense completed action.
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.
By way of analogy, a child can be taught “clean your room,” perhaps rewarded or punished when that does or does not happen. But eventually a child matures to the point where they understand living in their own filth is not healthy and it is better to spend a little time clearing up their room on their own without the threat of punishment or the promise of a reward. They may even begin to discover ways to keep the command “clean your room” which go beyond a child’s immature view of what the command meant. As a child matures they (should) move from shoving clothes under the bed for the appearance of neatness to shoving them into a drawer, to folding them and placing them in a drawer, etc. As a child grows towards maturity, they discover new ways to obey the original command, and often that command starts to make more sense when viewed with mature eyes.
In a similar way, as the one who is in Christ obeys the commands of God, they ought to grow more mature and find ways to apply the command to “love one another” which are less obvious or more challenging.
The last phrase concludes the section, “By this we know we are we are in him.” But how does this work out in real life? What are some real indications of growing maturity in the Christian life?