Christians’ believe that before Jesus mankind had committed so many sins (done so many bad things) that the people had become cut off from God.
They believe that Jesus’ death on the cross shows God’s love for mankind. It was the ultimate sacrifice for mankind.
God gave his only son, knowing that he would have to die, as part of God’s plan In effect Jesus (although innocent) took the punishment for mankind’s sins.
By doing this he cleansed mankind of sin and evil so that God could forgive it. Jesus therefore enabled mankind to again develop a relationship with God.
This re-connection is called “the Atonement.” (Atonement means, “making one”).