Why does Christ's death for all prove that all were dead in sin?
Christ's death was for the whole human race, not merely for one man or Israel. Since the entire human race needed His sacrificial death, it follows that the entire human race was dead in sin—separated from God because of Adam and unable to gain favor by works.
Les explains that the love of Christ constrained this man because Christ died not just for Saul, not just for Israel, but for the whole human race. Since the whole human race needed His sacrificial death, it follows that the whole human race was dead in sin. Les stresses that all are sinners by virtue of being born from Adam, not because of what they have done. We sin because we are sinners, and we are sinners because we are born of Adam's lineage, separated from God by a vast gulf. People who try to make eternal life theirs by works deny this, refusing to admit that they have been totally separated from God and are dead spiritually.