Jesus is called the Savior in Christianity. Literally, this word “savior” means that Jesus rescues us. So what did Jesus accomplish? He rescues us.
But what does Jesus rescue us from? Christianity believes that God made the world and all that is in it. God made everything good, with no bad whatsoever. But another spiritual power came in, and, somehow, by deceiving human beings, was able to introduce bad into the world. The goal was to destroy all that God had made, including us.
But God was not willing to let that happen. There was only one way God could stop it.
There was only one way God could defeat this evil power trying to destroy everything, through a process that had to start with God becoming a human being.
Why this was the only way God could stop the power of evil is something Christianity does not address. Neither does Christianity address why it required God to become a human being. Those are just accepted as givens in Christianity.
The point is that God had to become a human being in order to save us from the power of evil, so that’s what He did. From the moment He got here, as Jesus, this power of evil was against Him, working against Him. It worked through the Jewish leaders. They opposed Jesus from the start and eventually succeeded in having Him killed.
At that point, evil felt it had won. Its operatives, the Jewish leaders, had succeeded in getting rid of Jesus. Or so it seemed.
Jesus rose from the dead. The chains of death had been broken. Evil had not won after all. Its days were numbered, for this was the start of a process that would eventually result in its destruction. Evil would not be able to destroy human beings.
Jesus, in the same human body the Jewish leaders had killed but had risen from the grave, ascended into heaven. Jesus took us, our humanity, up there to heaven, and seated it right beside God.
And, one day in the future, when the process of the complete destruction of the force of evil is almost complete, Jesus will return here to earth, and there will be one final conflict between the forces of good and the forces of evil. The forces of good will win. A new world will be made, a world where all is good, where there is no evil, where there is no bad.
The fact that God came here in Jesus, was killed but rose from death, means human beings will not die. Our bodies will die, but we will be given new bodies that will never die. There will be no sickness, no heartache, no suffering, nothing bad at all. It will be the way God originally intended our lives to be.
This is what Jesus accomplished and is in the process of accomplishing now.
Keep in mind that Christianity uses human terms to attempt to explain something that is beyond the ability of human terms to explain. Christianity uses human terms to talk about the spiritual, but the spiritual is beyond the ability of human terms to describe. Human terms can only very crudely approximate describing the spiritual.