One of the most basic truths in God's program involves the fact that the wages of sin is death (Romans 6:23). The death we are intended to understand is the second death. There are only two ways to satisfy this basic truth: First, all humans must be paid that wage because all have sinned and come short of the glory of God (Romans 3:23). Second, another, an innocent One on whom death has no claim because He never sinned, must pay that wage in our stead, substituting His death for ours.
We find both aspects applied to practical Christian life in Romans. Paul writes in Romans 5:8, "But God demonstrates His own love toward us, in that while we were still sinners, Christ died for us." It is essential that we thoroughly understand that Christ died, not merely as a benefit, but for us, that is, in our place. His death substitutes for our well-deserved death, which we earned through sin. Earlier, the apostle had written in Romans 4:1-5:
What then shall we say that Abraham our father has found according to the flesh? For if Abraham was justified by works, he has something to boast about, but not before God. For what does the Scripture say? "Abraham believed God, and it was accounted to him for righteousness." Now to him who works, the wages are not counted as grace but as debt. But to him who does not work but believes on Him who justifies the ungodly, his faith is accounted for righteousness.
When confronted by such scriptures that cannot be broken, our only possible conclusion is that the sin-debt that each person owes to God absolutely cannot be worked off. It is so huge and serious that an already sin-defiled person cannot pay it off. Once a person sins, his debt is absolutely irredeemable by anyone or any action except through death. Either each individual pays for himself, or Christ pays in his place. These are the only acceptable payments.