When a person is obsessed by the idea that he or she has committed the unpardonable sin, such obsession is the result of Satan's attempt to harass a saint, rather than his effort to destroy a sinner. If that may seem a somewhat strange thing to say, I want quite simply to attempt to make clear what I mean by it. I speak now entirely from experience, and experience may not be trusted as infallible foundation for dogmatic statement.
Speaking entirely from experience, I declare that I have never yet found a man or woman, hard and rebellious and determined in sin, who possessed that particular fear. It is always the fear of the sensitive soul, always the fear of some trembling child of God. I do not say that it is always the case, but I do say that I have never met an exception.
Therefore, I have come to the conclusion that Satan never destroys men by making them believe that they've committed such a sin, but he does harass the saints by attempting to make them believe that. A method I have invariably followed for many years in dealing with those who come to me and say that they have, or that they fear they have committed the sin against the Spirit which has no forgiveness, is that of asking them this question: If you have committed this sin, will you be good enough to tell me what it is? I have never yet found a person possessed of the fear that they have committed it who could tell me what it is.
Adapted from Unpardonable Sin, by G. Campbell Morgan.