Many readers assume John is referring to the so-called deadly sins (murder, adultery, etc.) One problem with this is that there is no list of deadly sins in the Bible. In the Law there are several examples of sin which is committed with the full intention of breaking the law as “unforgivable.” Leviticus 4:2 for example, the one who sins with a “high hand.” Even someone like Paul who caused the death of Stephen found forgiveness from that murder.
Jobes points out that Jesus taught anger is as bad as murder, and lust as bad as adultery (Matt 5:21–22, 27–28; Jobes, 1, 2, & 3 John, 238). The gospels blasphemy against the Holy Spirit (Mark 3:28–30), but this is a particular rejection of the Messiah by the Pharisees when they attributed the work of Jesus to the devil.
It is possible John has recanting one’s faith in the face of persecution in mind, as in Hebrews 6:4-6. Persecution is not obviously in view, unless the reference to idolatry in the last line of the letter refers to the imperial cult.
Some have considered the “unforgivable sin “to be the act of suicide. Aside from the fact suicide is not addressed as an unpardonable sin anywhere in scripture, it seems highly unlikely John would say “don’t bother praying for someone who has committed suicide.” But John cannot have in mind physical death, because all people die whether their sins are forgiven or not.
Colin Kruse suggests John “very likely that he has the sin of the secessionists in mind” (Kruse, Epistles of John, 192; Jobes agrees, 236). Although it seems extremely strange to say, “don’t pray for those unrepentant sinners,” Kruse points out the prophet Jeremiah was told not to pray for Israel “because her sins were so repugnant (Jer 7:16-18; 11:14; 14:11).”
The one who has been born of God does not “keep on sinning” (5:18-20). This final paragraph returns to a theme found throughout the letter that the one who has been born of God does not continue in their sin. As we have observed at other times in the letter, John does not say that the Christian never sins, but they don’t persist in a continual state of sin.
John has already mentioned the power of the devil several times in the letter, but here he promises that God will protect his children for the power of the evil one (vv. 10-20). This is extremely important since John’s congregation is living in the city of Ephesus where the power of Rome was on display for all to see. It is possible John’s congregation feared the power of Rome as Christianity grew.
Taking the book of Revelation into consideration, this is not a promise the Christian will never suffer, but rather a promise that God will overcome the power of the devil. Even though the church is persecuted, and some may even die for their faith, they still do not fall into the power of the devil.
I think this “sin that leads to death” has something to do with the enigmatic final line of the book, “keep yourself from idols.” Perhaps John’s opponents are teaching their followers they can perform some kind worship of the Emperor or veneration of Rome to avoid persecution. This would be analogous to an American Christian pledging allegiance to the flag. Just as most Christians do not see this pledge as an act of idolatry, so too John’s opponents may have interpreted Imperial worship as an oath of loyalty and not actual worship of gods. They may have considered eating meat sacrificed to idols or attending meals or banquets held at local temples to be “not a sin.”
If this is on the right track, then the “sin that leads to death” is putting oneself in a place where they may not be sinning (yet), but there is a real danger of returning to the worship of idols. This is a very real problem for Christians living in cultures where gods are worshiped regularly. For Christians living in places which venerate ancestors, there is a very real struggle for the Christian to return to those practices in order to keep the pace within a family. The willful choice to return to idolatry is, for John, a sin that is so dangerous is “leads to death.”