One of the most difficult part to understand in Christianity is that Jesus, the son of God, died for our sin, so that if you believed in him, then you would be saved. This is summarized in the most popular quote from the bible, John 3:16, "For God so loved the world, that he gave his one and only son, that whoever believes in him should not perish, but have eternal life."
But why did Jesus has to die in order to save us? The classic explanation is that, God is both loving and just. Since the fall of the first man, Adam, we are all sinners. As God is just, we, as sinners, all need to be punished in eternal death (eternal separation from God). But God is so loving, he does not want us to be punished in that way. Meanwhile, God is just, so he cannot just forgive our sin based on his authority alone. Sin needs to have the consequence of punishment. When Jesus died on the cross, both justice and love are accomplished at the same time.
To many people, this explanation is still too abstract to grasp. More importantly, one piece that is missing in this puzzle is that, we need to love Jesus. Without such love of Jesus, justice is not complete, and salvation could be elusive.
First of all, sin as a result of free will - the capability of free choices. People might argue that God can create a world without sin. Let's just imagine a few scenarios. One scenario is God created only one person, so the only possibility is that person is to sin against God since there is really no other person to sin against. Or better, yet, that person is created in a way that can only say "yes" to God and never say "no". Will God be gloried with such a one-single-being world? Not so much. The second scenario is God created more persons but they all had to live alone to avoid sin against each other. This is not so impressive either as to glorify God. The third scenario, God created a world of many people in which they form communities and live together. To prevent sin, God can create a mechanism that whenever Cain wanted to sin
against Abel, Cain would have a serious headache or something like that. For example, right before Cain raised his knife and wanted to kill Abel, his arm spasmed and his body writhed and he had to fail his commitment of slaying by dropping the knife. The first homicide in human history can be thus avoided successfully, so can be all future crimes and sin. But what can be achieved by this? It is not a free world, it is a prison. With today's technology, maybe all we need to do is to force every one to wear a device and a monitor so that when crimes are detected, that device will stop our behavior so that sin cannot be committed, and in the worse case, to shoot oneself in order to prevent a crime of homicide. I am sure nobody will like to wear such a device. Everybody likes to live in a free world, in which we can make free choices about what we will do and not do, regardless it is good or bad. As a matter of fact, only when we live in such a free world, love can be developed and expressed in its full freedom and sincerity. Kids do not like parents to intervene their lives too much although they all know that parents have good intention - the so called parental love, to do so. So are we. Every day, we sin so much, every thought, every word we say, things we do and don't want to do, if for each such little sin, God gave you a nudge here and a poking there, don't you feel God is too annoying? He would be like a strict parent that is always around and criticize and correct everything we do. A loving parent only gives reminder and exhortation when necessary, allowing kids to make mistakes and have failures so that they can learn from those mistakes and failures. God is like a good parent and behaves in a similar way.
Second, when there is sin, there is suffering. The suffering comes from the interaction of imperfect people with imperfect people. God created humans and also like them to live together with love and support to each other. But love hurts and love heals. So, from the fall of the first man Adam, the humanity lives in a reality of triangle: love, sin, and suffering. However, God already have a world of only love in his design. In order to bring us to that perfect world, the solution is to give up his son, Jesus, to die for us, so that through his love and suffering, we can leave the world of love, sin and suffering, and enter a world of love only, the eternal life with God.
To do that, he cannot take away our free will, otherwise, all beings will become puppets, not a glorifying design. Instead, Jesus died on the cross for our sin and then was resurrected on the third day, overcoming death once for all. His love was demonstrated to us, and the justice was accomplished. God's wrath was inflicted on his one and only son, not on us anymore.
However, you need to love Jesus, as a response to Jesus' love for you. Only when you do this, his death and salvation are relevant to you. This is the mystery: you sinned, and because of your sin, you lost your loved one (Jesus), then justice has been done, because what can be worse as punishment for you than to see your loved one die because of your sin? You mourned over his death, you repented, and then you found your loved one was resurrected. Then you joyfully embraced him back, only to stay with him forever.
But if you treated Jesus like a stranger: you sinned, Jesus died for you. You think that is not a big deal since you don't love Jesus. So you don't mourn, and therefore, you don't feel like losing anything. His death is not a loss to you, and you are indifferent to his excruciating suffering on the cross either. With such indifference and treating Jesus as a remote stranger, you cut off the relevance of his death and thus his salvation from you. You stay in your sin and remain unsaved. Only Jesus's love can save you, but he has to be your "beloved" one to do the justice of your sin.
Of course, in reality, Jesus died first for all of us as when Jesus was on the cross, none of us has even been born yet. But God does not live in time, we do. God can foresee our birth as well as our sin. From God's perspective, it is not "I died first and then you were born and sinned", it is like "You were born and sinned, so I died for you." Holy spirit will reveal such God's perspective, and not only this, but also "If there is only you that were born and sinned, I would still have died for you".
So, instead of intervening our lives here and there, like a nagging parent, God did one of the most historic and significant intervention once for all: he died for our sin for love and justice and he was resurrected to overcome sin and death. Through this, a path of salvation was provided, one that brought us from a world of "love, sin and suffering" to a world of "love" only. Yes, God do want to create a world without sin and suffering. Actually, he is doing exactly that right now, only through a process. He is doing this not by just watching over humanity's suffering, but being part of that suffering
and death. So even in the worst part of our life, suffering and death, we can still feel God's presence. God knows what needs to be changed is not the outside of us (which he can do easily), but inside of us. God only works on one single humanity problem: How can a rebellious, perverse and restless heart be transformed into one that is holy, loving, submissive and peaceful? We prefer a design that takes suffering out of the equation, but God chose a design that allows suffering, both for humanity and his son, in order to nurture and develop such a heart. He saved each of us not despite
suffering, but through suffering itself.
Can we come up with a better design than this? Not really! After all, who are we to become God's counselor? As the bible says "Oh, the depth of the riches of the wisdom and knowledge of God! how unsearchable his judgments, and his paths
beyond tracing out! Who has known the mind of the Lord? Or who has been his counselor? Who has ever given to God, that God should repay him? For from him and through him and to him are all things. To him be the glory forever!
Amen." (Romans 11:33)