If the death of one man seems disproportionate to the sins of all of humanity, it should be remembered that only God can be the judge of Jesus’ value. The time factor of sins of the past and present is really irrelevant when it comes to this question, because God’s knowledge is not bound by time as our minds are. God, who is infinite, exists in the eternal “now.” He sees all and knows all without regard to time (Psalm 139:16). The issue is whether Jesus’ sacrifice could cover all of humanity’s sins. This is a question of his value. Jesus is the sinless Son of God (1 Peter 2:22, Mark 1:1). If he were not sinless, he would have died for his own sin, but because he is sinless, his death could make payment for the sins of others (Hebrews 7:26-27). Furthermore, though he is a man, he is not merely a man. He is infinite and eternal; He is, by very nature, God (Hebrews 1). He has both a divine and a human nature. Since he is human, he can represent humans on the cross. Since he is God, his value is infinite. Therefore, if a blood sacrifice can atone for sins, the sacrifice of Christ is of infinite value, and can atone for all sins to which it is applied. As God incarnate, his value is infinite. That should cover it.
Now, that raises a big question. How can such a sacrifice atone for sins? I will assume that the questioner is not a believer. If not, pardon the assumption. First, I would ask you to answer a question for me. How is it that the gray matter of the brain produces immaterial thought? Science is incapable of answering such a question, and there is no necessary logical connection between the brain and thought. If my point is not clear, let me state it plainly: the absence of a clear logical connection between phenomena is not proof that no connection exists. There is no clear logical connection, in my mind, between Christ’s death and resurrection and the atonement of any sin, let alone all sin.
The problem we have here is a question of truths of faith vs. truths of reason. Before you totally dismiss “truths of faith,” however, I am not talking about “God in the box” theology, in which we merely pull God out to answer anything we can’t explain. The faith I am referring to is not blind faith. In fact, we all use faith as a means of coming to knowledge. For example, when a doctor prescribes a medication, people commonly take the medication without understanding all of the details of their disease and how the medicine will combat those symptoms. Also, people judge every day that the airplane on which they are about to travel is sound based on the testimony of the authorities on the matter, not because they have first-hand knowledge, having investigated these claims. That the medicine will remove the symptoms of disease, or that the plane is sound is a matter of faith, but no less true than conclusions arrived at first-hand. A mentally handicapped man may not be able to comprehend square roots, but he would not be wrong to believe that three is the square root of nine based on the testimony of one more intelligent than he.
The question, then, is whether the authorities on the matter at hand are trustworthy. Assuming the New Testament accounts of Jesus are historically accurate (which is not the undertaking of this article to establish), Jesus himself claimed that he would be put to death and be resurrected (Matthew 12:40, 16:21; John 2:19-21), and that his death was, in fact, for the ransom (atonement) of sinners (Matthew 20:28). His disciples, the men closest to Jesus, having spoken with him after the resurrection, taught that his death and resurrection brought atonement (Acts 2:32-38, 4:10-12). Some truth is revealed, rather than attained through reason. If one will admit even the possibility of God’s existence, he must also admit that revealed truth is a possibility. If Jesus was indeed raised from the dead as predicted, we ought to believe him when he says that his death and resurrection bring salvation to all who receive him!
The fact that we cannot explain the “how” of the atonement does not make it false, any more than being incapable of explaining the connection between brain matter and thought makes it false. The apostle Paul said that the Gospel is foolishness to unbelievers (1 Corinthians 1:18). Indeed, this is true, since without the context of belief in God and in the resurrected Christ, the message of the cross just sounds like nonsense. Those who doubt the truth of Christianity would do better to spend their time investigating the existence of God, the historical accuracy of the New Testament, and the historical veracity of Christ’s death and resurrection, because once these conclusions are made, the atonement may be taken on faith. I would never ask someone to just believe it without any evidence. But, once the reliability of Jesus and his apostles to speak on spiritual matters is established, it is perfectly reasonable to believe what they say.
“If Jesus died and stayed dead for two days and rose on the third, how is that enough to atone for the sins of the past present and future?” I have no idea. But this doctrine was handed down by Christ himself, and by his closest companions. So I believe it.