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The whole issue of man’s eternal separation from God revolves around God’s holiness and man’s sinfulness. As Habbakuk. 1:13 states, God’s eyes are too pure to behold or approve evil and He cannot look upon sin. This does not mean God does not see man’s sinfulness, but that He cannot allow that sinfulness into His presence nor can He fellowship with such. But He is also a God of love and has made a way through the death of His Son that man might have fellowship with God. By faith in Christ, God imputes His righteousness to the sinner. In this way, God’s righteousness and justice are satisfied and He can receive and have fellowship with the sinner. Read chapters 3 and 4 in Romans.
Furthermore, the Scripture shows us that another part of the issue is that man is without excuse for remaining in his sinful state of separation from God. Read Romans 1:18f. If there is a real hunger to know God from the evidence in creation, God will get information to that person. See John 7:17, Acts 17:22-34.
But back to the original point. Revelation 20 speaks of a future judgment called the Great White Throne Judgment. This is an examination of all those whose names are not in the Lamb’s book of life, which is a record of all those who have put their faith in Christ (this means they stand justified and in the righteousness of Christ). Thus, the Great White Throne Judgment will be a time of examination where everyone who has rejected Christ (or the hope of Christ, as would be the case in Old Testament times), will be given their day in court where they are examined and the true condition of their hearts are brought plainly into view. Books are opened which contain a record of every person’s works (and this includes their motives). This will prove without a doubt that they are unrighteous and fall short of God’s glory and deserve eternal separation from God.
In his rebellion, man often cries out “unfair” or “unjust,” but I believe this is because of the fallen condition of man’s heart, which is naturally proud and rebellious, and because he simply cannot begin to fathom the absolute and awesome holiness of God. God’s holiness is emphasized in the Bible much more than is His love. God is light, which speaks of His holiness (1 John 1:5), and yet Scripture also says, “God is love” (1 John 4:8, 16) and He gave His only (or unique) Son because He loved the world (John 3:16). See also John 3:36. The point here is that God cannot and will not act against his holy character. In His righteousness, He cannot have fellowship with sin and in His justice He must condemn sin, act in justice. For instance, the Bible says God cannot lie. Why? Can’t God do anything? No, not because He does not have the power, but because His own holy character must always act consistently with His holiness. But God is also love, grace, and mercy, and because He always acts consistently with His character, He provided a means of salvation for sinful man through faith in His Son.
This was all true for Adam and all his seed. In Old Testament times, men were saved by faith in the promise of the coming of a Savior, even though the details of this were not as clear as they are for us today. This is why there will be degrees of punishment in eternity, which have to do with the amount of revelation one has had and rejected. Note what Christ said to the cities mentioned in the following verses because of the amount of special revelation they had seen and yet rejected:
Woe to you, Chorazin! Woe to you, Bethsaida! For if the miracles had occurred in Tyre and Sidon which occurred in you, they would have repented long ago in sackcloth and ashes. 22 “Nevertheless I say to you, it shall be more tolerable for Tyre and Sidon in the day of judgment, than for you.