Ok, I'm not a pastor, but I'll try to answer some of these questions as best I can. But like someone already said, you're asking a lot of deep questions that can't be answered on NS in 5 minutes.
First let me say that Jesus never said anything to the effect of "Jews will never be happy" or whatever someone has the notion of. In fact, he prayed "Father, forgiven them, because they don't know what they are doing" as he was hanging on the cross, he asked specifically for their forgiveness.
In the beginning when God created Adam and Eve, he walked with them in the garden every day,/i>...he created us to have relationship with him. Imagine you getting to walk with God every day.
Sin entered the world because Adam and Eve ate fruit from the tree of the knowledge of good and evil, up until then, they had no understanding of evil because they had never experienced it, they lived in a perfect garden, where they walked with God, etc.
The fruit itself wasn't evil, what made it a sin was that God specifically told them not to eat it, and they disobeyed, same thing as when your parents tell you not to do something and you don't obey. Now people have said all kinds of things about how God threatened them saying they would die if they ate the fruit, and why it was even in the garden...here is what I believe:
God didn't threaten, he warned them in love, it would be like your dad saying "there are cars outside on the street, and it's dangerous, you could get hurt, so I don't want you to play in the street"...he's telling you ahead of time so you know and you won't get hurt cause he loves you. But if he locked you inside all the time and never let you have the chance to go near the street or anything else that could hut you, he doesn't trust you. So God gave them the choice, because he trusted them.
Because of the sin though, God couldn't fellowship with them anymore, he can't be in the presence of sin or even look at it, he is too Holy. So for a long time people had to make sacrifices to atone for sins. And the sacrifice had to be a "spotless lamb" meaning a baby, lamb, white wool, nothing wrong with it, etc.
Flashing forward a bit:
Jesus Christ is born, lives 33 years on the earth and never sins, even when Satan in the flesh comes and tempts him. This makes him perfect. A spotless lamb.
Because of our sin, we have to be punished. God is a just God, and therefore if offenses have bee committed, he has to punish us. But because he is also a loving God and wants to have a relationship with us, he came up with a way to substitute Jesus in our place.
It's like an exchange, Jesus was perfect, we aren't...but when he died, he took all the punishment of sin on himself for us. When he was hanging on the cross and said "My God, my God, why have you forsaken me?" it was because at that moment all the sin of the world was on his shoulders, and God had to look away from his son, and Jesus couldn't see his fathers face.
But because he took all the sin, God counted it paid in full, as long as were are under his son's name.
Think of it like a restaurant...you go in and eat, and as you're leaving you need to settle your bill, but you have no money...however there's a big wedding party on the other side of the room, and the whole bill for their food is already paid for. If you were with that party, you would be covered. Well, that's basically what has happened, Jesus covered the bill, the only condition is we have to claim ourselves on the wedding party list. And we're invited.
Anyway, because God sees the people on the list under Jesus' name as paid for, he sees them as perfect, the same way his son was, and that means we can have a relationship with him again. It's like Jesus came down and said "I'll make you a deal, if you give up your coat of sin, I'll trade you and give you my coat of righteousness"
The reason he died was because that was our punishment, and he knew that the only way for us to be able to have a relationship with God was if we were perfect, and the only way for us to be perfect in God's eyes was to have our sins paid for by a perfect sacrifice.
This was kinda long, but there is so much more I couldn't put here, if you have any questions or want more information, PM me and I'd be happy to talk with you about it.