During His years of earthly ministry, Jesus preached exclusively to Israel. His message was essentially: "I'm the Messiah—the Son of Man of Whom the prophets spoke. Repent and be baptized, Israel, for the kingdom of God is at hand!"
If Israel had accepted Jesus' gospel of the kingdom, human history would have unfolded very differently. With Jesus as their king, Israel would have become a righteous, priestly nation. They would have preached to all nations a message of repentance and reconciliation to the One True God. And the Gentile nations would have been saved by faith in Jesus, the righteous king. That was Plan A.
But Israel rejected their Messiah. The Jewish leadership conspired with the Gentile Romans and crucified the promised king. God knew this would happen, of course. That's why the Father, Son and Holy Spirit had already devised Plan B from before the beginning of time.
This is Plan B: As Jesus suffered on the cross, the Sovereign Father poured out His wrath on the Son. His wrath was poured out to make payment for all the sins of all the humans of all times. Including me and you. Jesus paid off in full our sin debt. From God's point of view, all humanity now carries a zero balance. However, there's still one sin God is unable to forgive—the sin of unbelief. So if we choose to believe we: 1) didn't owe a debt for our sins, or 2) still have a debt to pay, God is legally obligated to keep our account indebted. Ultimately, He'll be forced to pour out His wrath on us. I don't think we want that, do we?
When the debt payment was completed, Jesus said, "It is finished." Then He died. He was buried in a wealthy man's tomb. After being dead for three days, He wasn't dead anymore. He was resurrected into eternal life, meaning He still lives right now. His physical body is different than ours, of course. It's incorruptible for one thing. And apparently He can move through space-time quite freely. Believers will get an incorruptible physical body like Jesus' to use in eternity.
What I've been calling Plan B is the gospel of God's grace (I Corinthians 15:1-4), preached primarily by Paul, the Apostle of the Gentiles. However, this gospel applies to everybody—Jews and Gentiles alike. It hinges on the death, burial, and resurrection of Christ. If you can believe this gospel of grace, God promises you will be saved (Ephesians 1:13-14).