Is there any sin so bad that God, through Jesus Christ, cannot forgive it?
Holy cow; I sure hope this is just a theoretical question!
Either way, it’s a trick question. Because the answer is yes—and, simultaneously—no.
So in the Bible Jesus names one sin—and one sin only—that cannot be forgiven. At Matthew 12:31-32 Jesus says:
“And so I tell you every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.”
For centuries theologians, philosophers, and others unsuited for normal employment have bent their minds trying to decipher what exactly Jesus is saying there. If Jesus and the Holy Spirit are one, they’ve pondered, how is it okay to blaspheme against one, but not the other?
Interesting question! Reason people go to seminary!
But I think the answer to this particular puzzler is positively easy.
I think that what Jesus is saying is that he understands perfectly well why some people will reject him. He has, after all, presented himself in mortal form—as the Son of Man—which he knows automatically renders subject to question the idea that he is in fact the creator of man.
See the trickiness? Having the only unpardonable sin be the rejection of Christ means that by definition no Christian can possibly commit that sin. And neither can any non-Christian, since you can’t reject from your life someone who’s never been in your life.
So the answer to your questions is that, yes, there is a sin that God cannot forgive—but it’s a sin that virtually no one on earth is capable of committing. Christians can’t commit it because they’re Christian, and non-Christians can’t commit it because they’re not.
As for Christians who renounced Christ, who are no longer Christian? Two things: 1. If they don’t care (and they can’t, since they no longer believe that Christ is any more real than the tooth fairy), then the question of what their new relationship is to Christ is the ultimate moot point; and: 2. As much ire as I know this will bring me [and it did: see below], my vote is that such a person was never really a Christian in the first place—by which I mean that their Christianity was always immature. And that’s certainly no crime.
What’s really interesting about Matthew 12:31 is that it says right there, in plain black and white, coming straight from the mouth of Jesus, that God forgives people who aren’t Christian.
[Update: I've shut down comments to this post because ... well, because how many times can I say that I didn't say what I'm being accused of having said?]