Q. Why did Jesus die for our sins? What would have happened if he hadn’t?
I was thinking about this question early this morning from 3:30 till 5:00 as my toddler was throwing a massive fit. For some reason, he had decided that sleeping in his crib for the rest of the night was not going to work for him. He didn’t like sleeping in the portable crib either (but his baby sister liked that better–he was out of the room they share). My prevailing thought was the importance of law, as a parent, to prevent chaos from ruling the house. (Bear with me, this will all tie together shortly. )
See, we have rules, as parents. Many of them we make up: you need to sleep in your own bed, you need to finish all of your food if you want dessert, you cannot hit anything but the floor, etc. Some of them are imposed by other people: you have to be buckled in your car seat when we are moving, you can’t take that candy from the store without paying for it, and so on. Because we are imperfect, lazy or tired, all parents choose to disregard the laws once in a while. “Fine, just get into bed with me and Daddy”, thinking that just for tonight, that will work. But of course, it isn’t just for tonight– it is now expected every night. “He’s starving–just give him Chex for dinner tonight”, but for weeks thereafter, when it is time for dinner, the toddler refuses to eat anything but Chex (or Goldfish crackers). When we disregard these laws, we lose the structure of our family society and everything can come unglued. Chaos reigns. Once you take a kid out of their carseat while driving they suddenly understand that what they thought was impossible is now possible. Same with Chex for dinner. Same with sleeping with Mom and Dad. There are no more rules and I can do whatever I want! It is a very dangerous place to be.
Luckily, our Heavenly Father has laws and He never deviates from them. Some of them He has created (the 10 commandments, for example) and some of them are just natural laws that He honors (gravity, rotation of planets, etc.). The law that applies to this question is the law of justice. Essentially, if you do wickedly, you will be punished and you are no longer worthy to return to live with God. If you do well, you will be rewarded. Unfortunately, every single one of us has done wickedly, and we cannot return to live with God–our righteous works don’t cancel out our sins, I think they are on separate balance sheets, as it were. So that is justice. It doesn’t help us at all, but it keeps the universe running smoothly. And God needs to honor justice fully in order to prevent chaos from reigning. As soon as He lets just one person in, with just the tiniest sin (like maybe, Moses or Abraham), there is going to be a big line of people demanding to get in too. They didn’t sin that much. They aren’t that wicked. Its a slippery slope.
God is an excellent parent, though. He can honor all of the demands of justice and still get us back to Him. His more excellent way is accomplished through Jesus Christ, who offers us mercy. When Christ came to earth, he was the only person in the history of this earth to ever live a perfect life. No evil thoughts, no evil actions, not even any selfish motivations. He lived perfectly. Before he died, Justice could have looked Him over and said, “Come on back in. You’re clean”. But Jesus didn’t just go back to Heaven after His perfect life–instead, He allowed himself to suffer for every person on this earth and every sin that they had ever committed. He died on the cross between two criminals after allowing himself to be beaten, scorned, mocked, spit upon, tried and convicted all by people like you and I. Regular justice-breakers. 
And in this way, Christ broke the law of Justice–but He broke it the opposite way that we break it. He broke it toward himself–He owned it. He should not have died and suffered for us in that way–that was not just. The atonement of Jesus Christ (this suffering and dying in order to reconcile us with God) was merciful and motivated entirely by love. Christ’s mercy can satisfy the demands of justice and still get us back to Heaven.
So, to answer your question: Jesus died for our sins to get us around the impassible heights of justice. If we ally ourselves with Him, He will vouch for us and His mercy carries us over the demands of justice back to our Father in Heaven. He is the only one who can do that. We can’t do it–we are imperfect. Heavenly Father can’t do it, He needs to honor justice (though He is, of course, carefully helping us find our faith in Christ so that we can come back to Him). Without Christ, we are subject to endless death and hell (an eternity without God).
For more information on this fundamental topic, see: