Romans 5:12-21 is pretty important for the second half of Romans 1 as is 1 Corinthians 15:20-24. I'll go to those in a moment, but back to Romans 1 for a minute. Adam and Eve knew God and this passage seems to refer to them and the curse we inherited through them, but they were only the first two among many. I think this passage makes a case for this. We're all guilty of becoming as they were: "futile in their speculations, and their foolish heart was darkened" (Romans 1:21, NASB). Wow! There's something very important in this passage which illustrates the point at which we're all guilty; not just Adam and Eve. Paul is talking about "them", plural. They, mankind became foolish in their speculations and their foolish heart (singular) was darkened. "Heart" is singular, meaning all mankind shares the same "heart" of hardness, unrighteousness, pride and suppression of truth. If we all share the same hearty of unrighteousness, we all share in the guilt and wrath of God which results. Taking this into account, this passage doesn't seem as harsh, for it says that all men have suppressed truth. Therefore, the condemnation of those which don't know God through Jesus is just. The Scripture tells us that if we're honest with ourselves, no one is searching for God and thus does not deserve Heaven. The message of the Scriptures is a God that searches out a rebellious mankind. Just as God sought out Adam and Eve in the garden after their sin, so God has sent his church, the body of Christ on earth, to search out the far reaches of humanity on earth to deliver the good news of Jesus Christ, the center and substance of the gospel.
God's "eternal power" and "divine nature" are understood through creation. What does the Scripture say that God requires as a result of this fact? The honor and glorification of the people, that is, that all people honor and glorify God based on His revelation of Himself in nature. If people sought out God on their own as we talked about above, then perhaps Jesus wouldn't be necessary, but the Scripture has already told us that all mankind shares the same heart which suppresses truth. Therefore, it's necessary for God to initiate salvation through Jesus Christ. We repent because of His initiatory kindness. It's because God first loved us that we can return his love. Jesus is God's physical manifestation of this truth. So, instead of honoring God as God based on His self-revelation in creation (ample proof that the visible is the evidence of the invisible), the Scripture says that people slipped into "futile speculation." This may seem hypocritical, but I'm going to ask a speculative question: If people were to honor God instead of speculate foolishly, would they go to Heaven or Hell? God puts this question to rest in Romans 3:11, reinforcing the point made above: "There is none who understands, there is none who seeks for God...". In other words, the situation which the question addresses will never happen because all mankind is rebellious. NOBODY honors God as God. All mankind, with the same heart, suppresses and hardens their hearts, futiley speculating instead of honoring God. We all continue what Adam and Eve only started.
Romans 5:12-21 gives some real insight into why all people suppress truth. God is not blaming us for Adam's sin. Rather, through Adam, sin which begets death entered the world. Sin spread to all men and all sinned, producing death. Verse 13 says, "...sin is not imputed when there is no law." This is great mercy on God's part, but there's one problem. Romans 2:14 already tells us that where the Law does not exist [meaning the Law of Judaism, Mosaic Law], it's known instinctively, written on the heart and so non-Jews are a law unto themselves through their consciences. Romans 5:14 tells us that Adam was a foreshadowing of Jesus, bringing death to all. Through Jesus, justification, grace and righteousness come. 1 Corinthians 15:20-24 basically echoes Romans 5:12-21.
So, why do we have to choose Christ by believing and repenting? Why doesn't His work come to us automatically just as death and sin came to us from Adam apart from our direct choice? Here's the difference: we are as Adam was, albeit cursed and fallen now. We were never like God or Jesus. Romans 1:21 makes it clear that we all share the same heart, even in our individuality. We all rebel, we all continue in sin. We are more connected than we may think. Practically speaking, if Adam hadn't eaten the apple, one of us certainly would have, and even this statement is a misnomer because it excludes Adam from our act of unrighteousness. If we're all unrighteous, the first one to sin had to be the first humans to have lived, Adam and Eve.