Jesus' life, death, and resurrection

To continue with the Creed:
... our Lord, who was conceived by the Holy Spirit, born of the Virgin Mary, suffered under Pontius Pilate, was crucified, dead, and buried. He descended to hell. On the third day he rose again from the dead, ascended to heaven, and sits at the right hand of God, the Father almighty, from whence he shall come to judge the living and the dead.
In this section the Creed talks about Jesus' birth, life, death, and resurrection. Christianity is based on historical events. The Creed is only a summary, so it doesn't give the entire history of God's dealings with Israel. But that is part of the basis for Christian belief.
One clarification is probably needed: non-Christians sometimes think that Jesus is like some of the pagan demigods, the result of a god having a child with a human mother. The Creed could be read that way. But that's not what it means. God is spiritual. He does not have sexual organs, and thus could not have a child in the physical sense. The Bible says that the birth was miraculous. Jesus' mother was still a virgin. Thus God was responsible for the birth. But not physically.
Jesus was executed by the Roman government, in a particularly gruesome way. However more than just the Romans were involved: he was betrayed by one of his own followers, and handed over by the Jewish authorities to the Romans.
Jesus had warned his disciples that he was going to be killed. He seems to have seen himself as carrying out a role described by the prophet Isaiah in a set of passages often calling the "Suffering Servant" passages. These passages described a person who would suffer on behalf of all of us, bearing the punishments that we deserved because of our sins. As a result, we would be reconciled to God. Jesus quoted Isaiah in discussions with his disciples. He was particularly explicit in the evening right before he was arrested, referring to his blood being shed to establish a new covenant, for the forgiveness of sins.
For a more complete discussion of this issue, please see the next page: the Gospel. It describes the reasons Christians see everyone as needing to be reconciled with God, and the way Jesus is seen has having done that. For the moment I will simply note that Jesus' death and resurrection are the key.
Jesus died, almost certainly on a Friday afternoon (although there are some oddities about the chronology as recorded in the Gospels). He was buried hastily, because the Sabbath (a holy day for the Jews) was about to start. On Sunday morning, a group of women came to the tomb, expecting to finish preparing his body. They found that it was no longer in the tomb. Jesus then began appearing to various of his followers, helping them understand the significance of his death and resurrection.
The term "resurrection" means coming to life again. Note that after the resurrection, Jesus seems to have had a somewhat transformed existence. It does not appear that his body simply came back to life. He was now able to pass through walls. However it was more than a ghost, or a vision. He ate a fish, and let someone touch him.