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.

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.”
Including the Spirit, there are three things are in agreement that Jesus is the Son of God. There are two possible allusion to the Gospel of John in these verses. In John 3:5, one cannot see the kingdom of Heaven without being born of water and the Spirit. In John 19:34, when Jesus is pierced water and blood flow from his side. When the soldiers approach Jesus, he is already dead but they confirm this by piercing his side with a spear. As a result, blood and water flow, indicating that the blood was already separating.
Some later manuscripts of the Latin Vulgate had “testify in heaven: the Father, the Word and the Holy Spirit, and these three are one. 8 And there are three that testify on earth:” These words were not in any Greek manuscript before the fourteenth century and did not appear in Latin Bibles until after AD 800. Likely this was a marginal comment which was introduced to the text by a scribe who thought it ought to be inserted. He was either mistaken, or thought it was a god place for a solid Trinity reference and he added it in.
Raymond Brown points out saying “God is love” is not the same as saying “God loves” (Brown, Epistles of John, 515). Since God is love, everything he does is an expression of his love. In this context, John mentions the coming Day of Judgment. Even rendering judgment and punishing those who remain in their sin is an act of love because God can only act in accordance to his loving character.
This is challenging since the world does not usually associate love with the organized church. Whether this is the classic cranky nun teaching in a Catholic school or an abusive priest, the puritanical Baptist pastor ranting against sin, preaching hellfire and brimstone on the street corner; the gossiping old women judging the way a younger woman is dressed in church; judging the kid with tattoos and piercings and wearing his ball cap in the church (backwards of course).