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.
Additionally, the Bible has much to say about the cross. The crucifixion is the vehicle by which God's mercy is made manifest to men. It's where Christ's blood was poured out that sins might be washed away. It's the focal point of salvation; it's the heart of the good news.
Peter spoke of the cross often. In his famous sermon given at Pentecost, he said this:
"Men of Israel, hear these words: Jesus of Nazareth, a man attested to you by God with mighty works and wonders and signs that God did through him in your midst, as you yourselves know -- this Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men."
The death of Christ is a part of "the definite plan and foreknowledge of God" -- it had to happen. There was no way around the cross. Jesus prayed in Matthew 26 for the Father to make another way possible and the events that followed make it clear that there was no other way. "My Father, if it be possible, let this cup pass from me," Jesus prayed (v. 39). It was not -- and is not -- possible for there to be a sacrifice for sins apart from the cross.
Elsewhere, Peter presented the cross in this way:
"For to this you have been called, because Christ also suffered for you, leaving you an example, so that you might follow in his steps. He committed no sin, neither was deceit found in his mouth. When he was reviled, he did not revile in return; when he suffered, he did not threaten, but continued entrusting himself to him who judges justly. He himself bore our sins in his body on the tree, that we might die to sin and live to righteousness. By his wounds you have been healed."
1 Peter 2:21-24
The place where sin was taken care of was the cross. The sins of man were placed in Christ's body on the cross as He suffered and died there. This sacrificial act allows for forgiveness, redemption, and reconciliation. Notice Peter's words: "By his wounds you have been healed." The wounds of Christ allow for sinners to be made whole and be reconciled to God.
Jesus died in our place for our sins that we might be made alive in Him. The cross is primary.