Adam and Eve disobeyed God and sin became their nature. Sin leads to death. Because God hates sin, he and sin can't co-exsist. Therefore any sinners can't be with God. This did not make God happy because he loves his creation and he loved us enough not to want for us to be seperated from him.
But How could he undo the sin nature? Adam was a man created without sin but given free will to sin if he chose. He did sin and was condemed to death and all his desendents inherit this sin. But if one of his desendents were to not sin and suffer death anyway, this would be unfair and a mistake. God would be guilty of allowing it. This can't stand because God is perfect and doesn't make mistakes. Therefore the original sin would be undone.
But how could a man not be born with sin? Sin nature is passed on through the father because even though Eve sinned, Adam was still incharge of her. The buck stopped with Adam not Eve. Because it is impossible for a child to be born without a father God had to find a way to have a baby desendent of Adam without a human father. God became the father (by way of performing a miracle IMO) and Mary, a virgin, the mother. Now baby Jesus is born without sin nature, but like Adam is tempted with all the same temptations. The difference is Jesus didn't take the bait from Satan and sin. Hence he died without sin.
Jesus is Gods son and therefore part of God himself, that is how the universe kept working when Christ died. Not all of God was in the grave. God is three in one. Father, Son and Holy Ghost
Why didn't he save his son from the hands of the Romans? That would have been counter productive as the goal was for an innocent man to die and undo the sin of Adam. Jesus Knew what was going to happen to him and agreed to it because it was the way to save man from seperation from God. He could have called it off but didn't. Three days in the grave is nothing in God time as 1000 years is a day to God. Christ, in God time, was in the grave for only seconds as far as God was concerned.
Now I have a Question for you. How can Muslims look at a man who killed,stole,and molested a child and say he is a man of God?