Why will people in the lake of fire have eternal regret?
People in the lake of fire will have eternal regret because they rejected the pardon. Their condemnation is rooted in unbelief, not merely sin, so they could have escaped if they had simply believed. They refused to hear, and though God has reconciled the world to Himself, they would not accept it.
Les explains that the lake of fire is different from hell, and those who end up there experience eternal regret because they rejected their pardon. They could have escaped it if they had simply believed, but they would not; they did not want to hear and were comfortable in their unbelief. Les emphasizes that lost people are not condemned primarily because of their sin, but because of their unbelief, though they will be judged at the great white throne according to the books. He points to God reconciling the world to Himself, not imputing trespasses, and uses Israel's rejection of Canaan at Kadesh Barnea in Hebrews 3 as a graphic illustration of rejecting God's promise.