At the Last Supper, when Jesus wanted to explain the significance of His death to His disciples, He referred back to the Old Testament. Jesus said in Matthew 26:28, “This is My blood of the covenant.” He was quoting Exodus 24:8, which is a reference to the covenant that God had made with Israel at Sinai, when Moses had taken the blood and sprinkled it on the people, saying, “Behold, the blood of the covenant, which the Lord has made with you. . .” In Luke’s description, Jesus says it a little differently: “This cup. . .is the new covenant in My blood” (22:20).
This language of “new covenant” is a reference to Jeremiah 31. But in both instances, Jesus is telling His disciples that what He is about to do on the cross is bring to fulfilment the covenant realities that Moses and Jeremiah had foretold. Surely, there is nothing more important for us to understand than the cross; and we cannot fully understand the cross apart from the covenants.
We cannot also fully understand salvation apart from the covenants: This is really clear from Romans 5. In Romans 5:12-21, Paul is teaching us about justification. And the way that he explains it is by setting forth Adam and Christ as two distinct covenant representatives. Paul tells us that we are justified in Christ in exactly the same way that we were condemned in Adam. And what we see is that God deals with men through covenant representatives. Adam represented all humanity, so that when he fell, all humanity was condemned with him. And it works the same way with justification.
Christ came into the world as the second Adam. And Paul demonstrates here in Romans 5 that just like Adam, Christ is the covenant representative of all those who believe in Him. So, just as all humanity was condemned on the basis of Adam’s disobedience, so too, all those who belong to the Saviour are justified on the basis of Jesus’ obedience as their covenant representative.