It is necessary, it seems to me, to begin from the fact that, yes, there is the unforgivable. Is this not, in truth, the only thing there is to forgive? The only thing that calls for forgiveness? If one is only prepared to forgive what appears forgivable, what the church calls “venial sin,” then the very idea of forgiveness would disappear.… There is only forgiveness, if there is any, where there is the unforgivable.
Source: On Forgiveness