Why would someone go to eternal doom if their sins are already paid for?
Their sins are already atoned for because Christ bore and God judged all sin on the cross; the one sin that sends a person to eternal doom is rejecting the Gospel. Eternal doom comes not from an unpaid sin debt but from refusing the salvation provided in Christ.
Les explains that Christ's crucifixion was not just another crucifixion, because He was the Creator God Himself who took on human flesh. On that cross God poured out all His wrath for sin, and all sin from Adam until the last person in time was laid on Christ and judged. Even the vilest lost and wicked people have their sins already atoned for and paid for. The only reason they would go to eternal doom, if they do not come into salvation in the meantime, is that they rejected it. Les teaches that the only sin that will ever put a person out in eternal doom is rejecting the Gospel, because the sin debt has been paid and finalized once and for all. This is why Paul burned until his dying day to get the Gospel of salvation out to lost people.