Understanding why Jesus had to die (Mark 8:31) is impossible without understanding God's plan to save His people. This plan was established in eternity past when the three persons of the Trinity covenanted together to redeem us; the Father would send the Son, the Son would die for His people, and the Holy Spirit would apply the Son's atoning work to believers (John 3:16; Eph. 1:3–14). But this plan was established in light of what God has ordained to happen in human history, namely, the fall of man (Gen. 3).
The violation of a covenant with the holy and loving Creator makes the fall a horrific event. In Scripture, we commonly find God dealing with people in terms of covenants, which are agreements between two parties in which each party pledges something to another. The biblical covenants are not made by equal parties—God imposes them on human beings. However, His grace is seen even in His making covenants with us, for strictly speaking, the Creator owes nothing to His creatures.
God's covenant with Adam represents the first of the divine-human covenants recorded in Scripture. Typically, we refer to this covenant as the covenant of works or the covenant of creation. Although such terminology is not found in the book of Genesis when it describes the pre-fall state, the various covenantal elements are present. Two in particular are worth noting. First, God promises to grant eternal life to Adam if he obeys the Creator and refrains from eating the fruit of the forbidden tree. We know this to be the case from Romans 5:12–21, which says that Jesus, the last Adam, secured eternal life for us by His obedience. It follows, then, that if the first Adam had obeyed, he and his descendants never would have died.
The promise of eternal life represents covenantal blessing, but covenants also have curses attached to them. So, the second element of the covenant of works is the curse of death God promises to Adam if he eats the forbidden fruit. Adam can eat of all the trees of the garden of Eden except the Tree of Knowledge of Good and Evil. In the day he eats of that tree, he will die (Gen. 2:15–17).
Theologians call the covenant with Adam the covenant of works because it is based on the principle of Adam's obedience. The blessing in this covenant is itself not a gift of grace that Adam does not earn. Instead, Adam must work; he must render personal and perfect obedience to the commandment in order to live.
God demanded obedience of Adam, and He demands obedience of us. However, our obedience as sinners is not what secures for us the new covenant blessing of eternal life. Instead, the obedience of Christ on our behalf is why we will live forever when we believe in Him. We are still called to obey, but now it is in order to thank the Lord for eternal life, not to earn it. Let us thank God for the gift of heaven by serving Him faithfully.