Christians believe that the human race is fallen in Adam. It is not as though the human race is sick and faces imminent demise because of the effects that sin will have upon us if we do nothing about it; rather, Scripture teaches that we are already dead in sins and trespasses (Rom. 5:12–19; Eph. 2:1). This means that we do not need a cure for the disease of sin before that disease kills us — it is too late for that. Instead, we need someone to pay for our sins, resurrect us from the dead, and give us new life. Unless and until this happens, we remain dead in sin, unable to do anything to help ourselves.
The Bible does not tell us how much time elapsed between the time God created Adam and the first man rebelled against Him in Eden. But the Bible does say that as soon as the human race fell into sin and received its wage, which is death (Gen. 3:14–19), God immediately promised to send a redeemer who would save His people, crush the serpent (Satan), and yet suffer in the process (v. 15). This wonderful promise is called the proto-evangelium (the first gospel announcement), and it is the basis for all of what follows in the Bible. In fact, from the third chapter of Genesis forward, all of what follows in redemptive history is the unfolding of God’s plan to save His people from both the guilt and the power of sin in fulfillment of this very promise.
The specific means through which God saves His people is that of a covenant. Not to be confused with a contract — such as the one you negotiate when buying a car or financing your home — a covenant is an act of God in which He swears in His oath that He will be our God and we will be His people (Gen. 17:7; Rev. 21:7). When God makes a covenant with His people, He promises them blessings on condition of their obedience. He also threatens them with curses should they disobey the terms of that covenant (for example, God’s commandments). The covenant oath is sworn under the threat of sanction (punishment). If God should fail to live up to His end of the covenant, the covenant curses threatened upon those who disobey Him will come down upon Himself. This also means that covenant-breakers must be punished. It is now a matter of divine justice.
This is why, in Romans 5:12–19, Paul speaks of two Adams. The first Adam sinned in Eden while acting as our federal (representative) and biological head. He was the first man (we all descend from him), and in his probation he represented all of us. As the puritans put it: “In Adam’s fall, sinned we all.” The guilt of Adam’s sin is reckoned to us, and the corruption that came upon Adam after the fall is passed down to us by natural generation. This means that we are born guilty for Adam’s sin and that we are born with a corrupt nature (for which we can also be condemned). This corrupt nature leads to a seemingly never-ending series of actual sins for which we must be punished. This is what it means to be “dead in sin,” and this is why we cannot save ourselves.
If any are to be saved (according to the promise of Gen. 3:15), either we must be punished for our own sins, corruption, and participation in Adam’s sin, or else someone must pay for these sins in our place. Since we are dead, we need a Savior — someone who comes as a second Adam, who obeys unlike the first Adam, and who serves as a covenant mediator, reconciling us to God. Such a person must be God so that the savior can actually pay this penalty and endure the wrath of God. But this savior must also be fully human so that the redeemer can bear the guilt of our sins for us and in our place, just as Adam acted on our behalf in Eden.
After raising the need for such a covenant mediator in questions 12–17, the Heidelberg Catechism identifies the covenant mediator as Jesus Christ, the second Adam. Question 18 of the catechism teaches us just this — that this covenant mediator is none other than Jesus Christ (1 Tim. 2:5).
As God’s eternal Son, Jesus took to Himself a true human nature, and He came to earth to save us from our sins. Supernaturally conceived in the womb of the Virgin, Jesus lived a sinless life, was unjustly condemned, and went to Calvary’s cross for the sole purpose of reconciling Adam’s fallen children from the guilt and power of sin. Laying down His life for us (“freely given”), Jesus bore the covenant curse, thereby satisfying the justice of God, so that we are completely redeemed from our sins and reckoned righteous.