Adam and Eve sinned, generously providing the rest of humanity with Original Sin and also the concept of sinning. Humans really like to sin, and continue to do so all on down the line for hundreds of generations. Before the great heavenly war began, before Lucifer ever even had an inkling that he would turn against God, God knew that His people would need saving at some point. He says that a part of Himself (the Son, according to John Milton) will one day go down to Earth and sacrifice Himself to save humanity.
In the year 1 CE, Jesus is born in a little town called Bethlehem. Jesus lives and preaches locally for roughly the next thirty years. His message is, largely, that people should “love thy neighbor,” i.e. be kind to one another. Gradually, Jesus amasses followers. The twelve most dedicated of his followers are known to us as the Apostles, one of which is Judas Iscariot.