Definition: Jesus, who is God, died on the cross in the place of man, who is utterly sinful, meeting the demands of God's justice and wrath because man could never do it himself.
Why it's primary: First, please note that I've written fuller thoughts on this subject in a book that's free to you. Subscribe to my website on the right side of your screen, and you'll receive a link in your e-mail.
All of history points to the cross. Events in the past pointed toward the coming of the Messiah and His death. Events in the present and in the future flow out of the once-for-all sacrifice that changed the world forever. When Jesus took on death, everything changed.