God always begins with the root of things, not surface matters. If the root is right, the leaves will grow green and the fruit will come. The cross deals with the root problem of mankind. What is that problem? It is that at the beginning, Adam sinned and led the whole human race into death. "And the Lord God commanded the man, saying, 'Of every tree of the garden you may freely eat; but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die'" (Genesis 2:16-17). The serpent told Eve that she would not die, but he deceived her. When Adam and Eve disobeyed God, they both died, as God said they would.
Since Adam and Eve lived on earth for hundreds of years after this event, how did they "die"? The use of the word "death" in the Bible is not limited to physical death of the body and does not necessarily mean non-existence. Before Adam and Eve disobeyed, God used to come every day to walk and talk with them. After they disobeyed, when God came to spend time with them, they found that their relationship had changed. Instead of welcoming Him, now they hid from Him in shame. Then they were put out of the garden (separated from God) and the process of physical death began in their bodies. They were now "dead" to God, and this death spread to the entire human race. "Therefore, just as through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned--" (Romans 5:12).
Thus, every descendant of Adam is born in "spiritual death"--separated from God, without the fellowship and communion with God that He intended and desired. Separate from God and His grace, sin reigns supreme (Romans 5:21). The essence of sin is self-rule--"I am my own god and I will not have any other tell me what to do." In his state of sin and death, man walks by his own light, his own reasoning, strength, and wisdom--all for his own purposes and desires. His desires and the influences of this world rule him. He is bound, unable to free himself. He cannot please God, and any attempt to be righteous is like "filthy rags" to God. He cannot lift himself out of this condition. He cannot give himself life to enter back into fellowship with God, any more than a dead man can rise from the grave by his own will. He needs someone to rescue him from this state of death. This is the gospel message: how God can bring a man, woman, or child back to life and into fellowship with Himself. This is the only solution, and it took the death of Jesus Christ to make it possible.