I believe the unpardonable sin is not possible to Christians because it is only in accepting or denying the prompting of the Holy Spirit to accept Christ that one comes across that situation and if one is a Christian, one has accepted the Holy Spirit to dwell within one. But if one has not accepted Christ as one's Savior and the Holy Sprit is prompting one to do so and one flat out refuses that counsel, knowing what one is denying, then one has indeed committed the unpardonable sin of denying God.