The reason God would punish someone for eternity for temporal sins committed on earth is based on who the sin is against, not on the size of the sins. Let me illustrate. If I were to walk up to you and slap you, you might get angry or even take a swing at me. We’re done (hopefully). If I were to walk up to the President of the United States and slap him, I would be arrested and probably spend years in jail. Why would the exact same “small” offense get such a different result? It is because of who it is against. Likewise, when we move up the chain of command to the infinite God of the universe, it isn’t difficult to see that the consequences are amplified infinitely as well. Therefore, eternal punishment in hell fits the crime--it is against.
Like it or not, when we sin, we sin against God. He is holy and He is righteous, and anything that we do that is contrary to His will--breaking the Law of God (1 John 3:4)--properly deserves punishment. Since our offense is against an infinitely holy God, the consequence is likewise infinite, and that is why eternal hell exists.
But also, what about those who have never heard the gospel or who have committed sins that are not aware of? It is, so to speak, not their fault. Why should they suffer such eternal torment for ignorance and accidents of there circumstances?
Biblically speaking, the standard of holiness rests with God, not with our ignorance and not with our circumstances. God says to be holy because He is holy (1 Peter 1:16). The proper standard of righteousness is based in God and is not altered by our situation. Therefore, those who are ignorant of God don't have an excuse. Furthermore, we know this is the case because of what it says in the Bible.
Romans 1:20–21, "For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse. 21 For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened."
God is telling us that the knowledge of whom He is is evident around us and that what we ought to do is recognize His existence and give Him glory. So, it doesn't matter what situation a person might find himself in. What is around him in creation is sufficient to inform him that there is a true and living God. Therefore, he is without excuse.
But some might object by saying that the Bible says you cannot be saved without knowing Jesus (John 14:6). So, if people have never heard of Him, then they are still in trouble. They are lost.
To this, we can say that God has his ways of communicating His presence to people around the world. Though salvation can only be found in Christ, there are plenty of instances where God has revealed himself to those who do not have the "normal" revelation of Scripture. Presently, in the Middle East, there are reports of thousands upon thousands of Muslims having visions and dreams of Jesus, and they are coming to the belief in Christ. Take John 4 for example, when Jesus was traveling north in Israel, went out of his way in order to meet the Samaritan woman and minister to her. In other words, God has his ways of meeting people, and we ought not to be so narrow to think he cannot reach anyone in the world.
There is one more factor worth mentioning. The biblical teaching of Federal Headship is that the male represents the descendants. Adam represented his descendants, and when he fell, we fell in him (1 Corinthians 15:22). In other words, we inherited a sinful nature, and that is why we are by nature children of wrath (Ephesians 2:3). So, the natural state of all people, because of their sinful nature, is to be separated from God (Isaiah 59:2). But, God, in his mercy, sent Jesus the Son, so that people might escape the necessary and righteous judgment of God upon the sinner.
God and the Law
Because God is holy and He is perfect, He requires holiness and perfection. When He tells us not to lie and steal that is because He cannot lie or steel. These things are wrong because they go against God's nature. If anyone lies or steals, he has sinned against God by "challenging" the holiness of God when he does that which is contrary to God's nature. If God does not deal with that "sin challenge," then God is being unjust by allowing that sin to go unpunished. So, God must punish the sinner because the sinner has violated his holy character and if God does not deal with it, then God is approving of that which is not holy, which would be a self-contradiction. God cannot contradict himself, and God cannot approve of unholiness.
So, judgment is a necessary effect of our sin against God.
But, God sent the Son to bear our sin in his body on the cross (1 Peter 2:24) so that we might be declared righteous by faith (Romans 3:28; 5:1). In other words, by trusting in what God has done on the cross and where He fulfilled the law perfectly (1 Peter 2:22), then when we trust in that sacrifice on the cross, we are considered to have fulfilled that law perfectly also because the righteousness of Christ is given to us (Phil. 3:9). That is how are saved from the righteous judgment of God.
What is sin?
Sin is breaking of God's law. It is a transgression, an iniquity, an unrighteous act. Sin is a deviation from the will of God. It is a form of evildoing since it is in opposition to God's decrees and desires. It is not a merely a deficiency. Sin is anything that is contrary to the law or will of God. Everyone, both Jew and Greek, is under sin (Rom. 3:9). Yet, "while we were still helpless, at the right time Christ died for the ungodly." (Rom. 5:6).
Federal Headship is foreign to the modern mind, but it is a biblical concept. It is the teaching that the father is the one who represents his family, his descendants. Proof of this can be found in Heb. 7:8-10.
What is salvation?
Salvation is being saved from the righteous judgment of God upon the sinner. Al of us have sinned against God and deserve judgment. But Jesus never sinned (1 Pet. 2:22). He lived the Law of God perfectly. In this He has a perfectly righteous standing before God. When the corrupt Jewish leaders forced Rome's hand into crucifying Jesus, God used this crucifixion as the means to place the sins of the world upon Jesus (1 Pet. 2:24; 1 John 2:2). This is when Jesus became sin on our behalf. 2 Cor. 5:21
Sin and Punishment in the Bible
What Jesus said:
Matthew 9:2–7, "And they brought to Him a paralytic lying on a bed. Seeing their faith, Jesus said to the paralytic, “Take courage, son; your sins are forgiven.” 3 And some of the scribes said to themselves, “This fellow blasphemes.” 4 And Jesus knowing their thoughts said, “Why are you thinking evil in your hearts? 5 “Which is easier, to say, ‘Your sins are forgiven,’ or to say, ‘Get up, and walk’? 6 “But so that you may know that the Son of Man has authority on earth to forgive sins”—then He said to the paralytic, “Get up, pick up your bed and go home.” 7 And he got up and went home."
Matthew 25:46, "These will go away into eternal punishment, but the righteous into eternal life.”
What Paul Said:
Romans 2:12, "For all who have sinned without the Law will also perish without the Law, and all who have sinned under the Law will be judged by the Law."
Romans 3:20, "because by the works of the Law no flesh will be justified in His sight; for through the Law comes the knowledge of sin."
2 Thessalonians 1:6–9, "For after all it is only just for God to repay with affliction those who afflict you, 7 and to give relief to you who are afflicted and to us as well when the Lord Jesus will be revealed from heaven with His mighty angels in flaming fire, 8 dealing out retribution to those who do not know God and to those who do not obey the gospel of our Lord Jesus. 9 These will pay the penalty of eternal destruction, away from the presence of the Lord and from the glory of His power,"
What the Old Testament Says:
Genesis 2:16–17, "The LORD God commanded the man, saying, “From any tree of the garden you may eat freely; 17 but from the tree of the knowledge of good and evil you shall not eat, for in the day that you eat from it you will surely die.”"
Psalm 32:1, "How blessed is he whose transgression is forgiven, Whose sin is covered!"
Damnation: "In general, ‘condemnation’, but esp. to eternal loss (damnum) in hell. Some theologians have distinguished the loss of the Beatific Vision, the attainment of which constitutes the joy of heaven, from positive retributive punishment for the sins done on earth, and asserted that both are present in the sufferings of the damned; others have held that the deprivation and conscious loss of heaven and of God is itself the natural and sufficient consequence of persistence in sin. The eternity of damnation for those who finally reject the will of God appears to follow from various sayings of Christ, e.g. Mt. 25:46." (Cross, F. L., and Elizabeth A. Livingstone, eds. The Oxford Dictionary of the Christian Church. Oxford; New York: Oxford University Press, 2005)