Remember that this isn't totally accurate. Think of it as a different version, or an allegory of the real story.
Imagine that you are God. You made some clay dolls and you brought them to life. You made them to bring you glory and praise. And even before you made them, you made a beautiful world for them to live on. (a perfect world I might add) You had given them only one rule to follow: they couldn't eat the fruit of just one tree, the tree of the knowledge of good and evil.
So they lived on that perfect world and worshiped you because you were their creator and they loved you.
One day your arch enemy visited them and told them all they had to do to be like you, God, was to eat of the forbidden tree. They said to your enemy, "No, God told us not to. He said that if we ate of it we would die." But your enemy lied to them and told them that they wouldn't die. So instead of choosing an amazing life with you, they ate of he fruit. Now because you were perfect and holy, sin couldn't be in your presence, so you had to curse them because of their sin. It broke your heart to do so, but it would go against your character to not punish sin.
A long time passed and sin just kept growing and getting worse in the people's descendants. After a long long time you decided to do something about it.
You had your one and only son go to the people on the world, and live like one of them. There was a difference though, your son wasn't sinful, he was perfect just like you. So anyway, your son lived a perfect life on that no longer perfect world.
There were a lot of people on that world that didn't like your son though, they wanted to kill him. They eventually managed to do so; they killed him, accusing him of crimes he didn't do. It broke your heart beyond imagination to see him in so much pain, but it was all according to your plan to save the people you loved.
The punishment for sin was death, and by having your son die in their place was literally the perfect sacrifice.
You see, until then your people had been sacrificing animals to pay for their sins. This did work mind you, but since it wasn't perfect, they had to keep sacrificing over and over. By your son being the perfect sacrifice, it counted once and for all.
It didn't stop there though. Your son even defeated death by coming back alive from the dead. Now all your people had to do to be with you was believe that they couldn't save themselves and that your son died in their place; they didn't have to be eternally separated from you anymore!
I hope you liked the story. Again, please remember that it isn't 100% accurate; I'm not perfect. You can probably tell what it is really about. (well duh, the story of creation etc.) The whole story is found in Genesis and the Gospels.
Please review and tell me what you think!