Why is Israel's rejection of the Holy Spirit the unpardonable part?
Israel's rejection of the Holy Spirit is the unpardonable part because after the nation rejected the Father by killing the prophets and rejected the Son by killing Christ, the Holy Spirit was the remaining person of the Trinity. The first two rejections could be forgiven, but Israel's treatment of the third person could not.
Les explains that Israel had already rejected the overtures from the Father by killing the prophets and rejected the overture of the Son by killing the Christ. Then he asks how they will deal with the Holy Spirit, because that is the unpardonable part. It is how they deal with the third person of the Trinity. Les says God could forgive the first two, but not really the third. This unpardonable sin deals with the nation of Israel coming under the anathema of God, not the one sin that condemns individuals—unbelief concerning the gospel. Les maintains someone could blaspheme the Holy Spirit, and God could still save him tomorrow or next week, because he is still a candidate for the grace of God, but the nation's rejection of the Holy Spirit is the unpardonable part. The context then moves to Acts chapter 6 and the appointment of seven men normally called deacons.