What responsibility do believers have toward people who have not received the pardon?
Believers are responsible to let people know they do not have to go to that awful place or be separated from God, because their pardon is already done. They should urge them to believe it, so they avoid eternal regret and doom.
Les explains that the eternal doom of those who reject their pardon is so horrible because for jillions of years they will regret, 'I rejected my pardon.' That understanding makes it the believer's responsibility to let people know they don't have to go to that awful place and don't have to be separated from God. The pardon is already done; they must believe it. Les also notes from Hebrews 3 that those who did not enter God's rest were those who believed not, not merely the immoral. He warns against the idea that a person must first clean up their act or change their ways to become a Christian; rather, one comes while yet ungodly.