"There is a sin that leads to death. I am not saying that you should pray about that" (TNIV).
I continue to think a lot about apostasy. No, not as an option for me (!), but trying to make sense of the experiences and decisions of others. I also recently finished Robert Yarbrough's new Baker Exegetical Commentary on 1-3 John and gave it a glowing review (see under Denver Journal on our website). John has a lot to say about the topic in these little letters and the verse quoted above may be the most well known of all he has to say.
Ironically, his main point in this context is to encourage his congregations to pray for those who have committed all other kinds of sins besides the one that leads to death (vv. 1 John 5:16-17). But by setting up the contrast between the two kinds of sins, he naturally piques our curiosity about the more heinous of the two.
It is unlikely that John is talking about sins that lead to physical death, as with Ananias and Sapphira in Acts 5:1-11. Every other use of "life" (zoÄ“) or "death" (thanatos) in the Epistles of John refers to spiritual life or death. In light of 1 John 2:19, it is unlikely that John thinks of these people who sin unto death as ever having been true Christians, though they may have fooled others and even themselves (the kind of deceit that should preclude us ever treating ‘eternal security" glibly or casually and that should ever keep us pronouncing with 100% assurance on the spiritual condition of anyone else).
1 John 3:10 offers us considerable help here: "This is how we know who the children of God are and who the children of the devil are: Those who do not do what is right are not God's children; nor are those who do not love their brothers and sisters." No, the kind of help I'm thinking of is not what some might immediately think of - that professions of faith must be complemented and thereby demonstrated by love and obedience to the commandments, though that is a central theme of John. Rather it is the simpler but subtler observation that those whom one category of "fake Christians" fail to love are called adelphoi ("brothers and sisters," or "siblings" for those who prefer an accurate, one-word English equivalent).
But the way this term of biological or spiritual kinship is used involves reciprocity. I never call someone my brother who cannot in turn call me his brother. So that means that the fake Christians in John's community would have also been called brothers (or sisters). Thus when this same language of siblingship reappears in 1 John 5:16, we dare not assume that it proves John has true believers in mind. He is simply echoing the language of the community itself as they refer to one another as brothers and sisters. Tragically, some who have these terms applied to them and perhaps apply them to themselves as well may turn out to have been masquerading, wittingly or unwittingly.
Are we therefore never to pray for such people? As Paul would say, mÄ“ genoito ("by no means," or for Denver Seminary grads who had Elodie Emig or me for Greek, you'll know the more accurate though dynamically equivalent translation that might offend some readers)! One has to recall that Greeks didn't put their negations in misleading places in their sentences like we do. John very intentionally says that he is not telling them, on this occasion, to pray for those who sin unto death. This is quite different from him telling them not to pray for them! He's simply saying that he's not talking about the sin unto death in this context but those sins that aren't unto death.
Of course, if we knew who those people were who had so hardened their hearts that they had committed what Jesus calls blasphemy against the Spirit (Matthew 12:32) so that God gives them over to their depravity (Romans 1:24-28), we could stop praying for them, knowing it was pointless. But we don't have such knowledge and when we guess as to who such people might be we often guess wrongly. So we dare never stop praying for anyone no matter how much it seems like they might be sinning unto death. Deathbed conversions remain surprisingly common even today, including by some of the once-most-hardened atheists and "believers"-turned atheists!
So what is the sin leading to death? Yarbrough puts it well: it "is to have a heart unchanged by God's love in Christ and so to persist in convictions and acts and commitments like those John and his readers know to exist among ostensibly Christian people of their acquaintance, some of whom have now left those whom John addresses" (p. 311). The assurance John offers is always for those who are presently believers (1 John 5:13), not for those who have repudiated their professions of faith. But as long as the breath of life remains in a person, repentance unto eternal life is always possible. The only unforgivable sin is the sin of unwillingness, in the final analysis, to repent and come to Christ.