Why did Paul labor so relentlessly to get the Gospel out to lost people?
Paul labored relentlessly because Christ's cross fully paid the sin debt for all people, even the vilest lost sinners. Since their sins are already atoned for and only rejecting the Gospel brings eternal doom, Paul burned until his dying day to get the Gospel of salvation out to lost people.
Les explains that Jesus' crucifixion was not just another crucifixion among thousands; it was unique because the Creator God Himself took on human flesh and went to the cross. There God poured out all His wrath for sin, laying on Christ the sin of everyone from Adam to the last person who will ever live, and judged it. Even the vilest lost and wicked people have their sins already atoned for and paid for. The only sin that sends a person to eternal doom is rejecting the Gospel, because the sin debt has been finalized once and for all. That is why Paul burned until his dying day to get the Gospel of salvation out to lost people; if any of us gets a part of that, we would make a difference.