Not only is the verb for sinning common in 1 John, so is the noun. The blood of Jesus Christ cleanses us from sin (1:7). If we say that we have no need of this cleansing, the truth is not in us, and the sin we deny we have is in us (1:8). But if we confess our sins (1:9), acknowledging that God is true when He labels our sin as sin, God is gracious to forgive us for those same sins (1:9). The reason John is writing to these people at all is because their sins are forgiven (2:12).
Christ averted the wrath of God, a wrath that was resting upon this world because of sin (2:2). This averting of wrath, this propitiation for our sins, is the demonstration of God’s nature as love (4:10). There is no tension between propitiation and love — the former is the display case of the latter.
What is sin anyway? Whoever commits sin is someone who has transgressed a boundary of the law (3:4), and we know that this is so because sin is the transgression of the law (3:4). Jesus Himself had no sin (3:5), which is why He was in a position to take our sins away (3:5).
The one who commits sin is of the devil (3:8), and is exhibiting a clear family resemblance to his father. The one who is born of God does not have this family resemblance any longer (3:9).
If we see a brother sin a sin not unto death, we should pray for him (5:16). If it is unto death, we should not pray (5:16). All unrighteousness is sin (5:17), but some sins, however unrighteous, do not result in death (5:17).