Why did the love of Christ constrain Paul?
The love of Christ constrained Paul because Christ died not merely for Saul or Israel but for the entire human race. Since all people were dead in sin and needed His sacrificial death, Paul recognized the scope of that love and it compelled him.
Les explains that the love of Christ constrained Paul because Christ died not just for Saul, not just for the nation of 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. This ties back to being born from Adam and separated from God; people sin because they are sinners, not the reverse. Those who try to earn eternal life by works deny this total spiritual separation.