This question comes up often and regularly when you are a pastor. Often, it comes form people worried they have committed it. Usually, these are not people who are in any great danger.
Before we get to what it is, it seems worth saying there is a basic principle of biblical interpretation that helps us understand it. Namely, we interpret the less clear by way of the more clear. We take the utterly clear parts of scripture and we read the more controversial and less clear parts in light of them.
When we apply this principle, one thing becomes clear. The bible is quite clear that any sin can be forgiven if it is repented of and brought to the foot of the cross. There are no sins that cannot be forgiven by Jesus except those that are neither confessed nor repented.
Even here, we need to be careful to remember that it is the Holy Spirit who brings us to repentance. But, let’s just be clear: the only sins that remain unforgiven by Jesus are unconfessed, unrepentant ones. And we must be clearer still, we are not implying a Roman Catholic view of forgiveness here. The Roman Catholics are big on giving people the last rites specfically because of fear that failing to repent of all sin will cause the believer issues before the Lord. Protestants reject this view and insist the one who puts his faith in Jesus, repents and confesses his inherent sinful state and receives forgiveness for all his sin; past, present and future at that very point.
If these things are true, where does that put us with respect to the unforgiveable sin? Two things are worth repeating. First, there is no sin that cannot be forgiven except that which is unrepentantly induged. Second, those who trust in Christ have all their sin – past, present and future – forgiven the moment they believe. These two things are quite clear from scripture. Given those things, the unforgivable sin must involve a) unrepentance and b) having not had all sin forgiven in Christ through belief.
When we think of it in those terms, the unforgivable sin most likely concerns someone who has neither confessed nor repented of the sin of unbelief. It is the sin of rejecting Christ and being utterly confirmed in that unbelief so that one’s sin simply cannot be forgiven.
If this is right, genuine believers really have nothing to worry about. If they have repented of their sin and trusted in Jesus, they necessarily haven’t committed the unforgivable sin. Their sin has, indeed, been forgiven by Jesus.