ANSWER: Some think it is the culmination of a long period of resistance to the Holy Spirit, a fixity of sinful character towards which all sinners are steadily drifting. When this point has been reached the Spirit abandons the soul (Isa. 63:10). Others say that the unpardonable sin is ascribing Christ's miracles to the devil, a sin which only those who lived when Christ was on the earth would be apt to commit. See Mark 3:29,30. Others think that when after a course of sin some aggravating insult is offered to the Spirit, most sensitive Person of the Trinity, then does he resent it with a justice that knows no mercy. A good illustration of this sin is this: There is a fatal disease for which there is but one cure. One finds the remedy; another compounds it, and the third applies it. Neither of the three will do the work of either of the others. If the sick man refuses to have the remedy applied, but trusts in a general way to the kindness of either of the others, he must die, though the supply of prepared medicine is ample. The same dreadful result would follow, if, because of some great insult to the third man, a philanthropic physician, he should decline to medicate the blasphemer. (2) He need not have been a Christian, though a Christian may commit this sin. (John 5:16.).
— Steele's Answers pp. 256, 257.