The answer to the question of why we, who were not present in the garden of Eden and did not commit the first sin of eating the forbidden fruit, still suffer the consequences of Adam and Eve's sin, is multifaceted.
I did not eat the fruit in the garden of Eden so why do I suffer the consequences of Adam and Eve's sin?
First of all we need to understand that Adam, as the first man, was God's designated representative of all who would descend from him. The theological term for this is federal headship. Adam acted as the representative of all of his progeny. Therefore, the Scriptures speak of all of mankind as being in Adam (1 Corinthians 15:22). Our modern individualistic mindset may balk at this notion. However, we would do well to remember that we did not and would not exist apart from Adam. We are all descended from him (Acts 17:26). Without Adam, we would not exist. It is impossible for you or I to have been tested as Adam because we did not exist. There is no pre-fall you or pre-fall me. There are myriad of other things that we inherit from our ancestors that we had no say or control over. Does this make God unjust? Of course not. We are mere created beings and God is the sovereign Creator (Romans 9:20). What have we given to God that He owes us anything (Job 41:11)?
We may vainly imagine that if we were in Adam's position then we would have chosen to obey and not eat the fruit. However, that position is misguided on at least two accounts. Firstly, Adam, having been born in paradise and without a predisposition toward sin, was in a much more advantageous position to resist than we are now and yet he succumbed to Satan's temptation. Given similar circumstances, why would we assume we would be any different from Adam? Secondly, God is omniscient. Therefore, He knows not only everything that was, is, and is to come; but He also knows everything that is possible. If it were unjust for God to allow Adam's descendants to be punished for his sin then God would not have done it. If it were possible that a sinless you or I (hypothetical and imaginative creatures) were to have remained obedient to God in the face of temptation then God would not have allowed us to be punished for Adam's sin. However, this is to enter the realm of the absurd because there cannot logically be descendants of Adam who preceded him. We are what we are, not what we wish we had been. Just as we had no control over our physical characteristics, our mental capacities, our place of birth, and numerous other things; we have no control over the fact that we are born with a sin nature (Psalm 51:5; 139:13–16).
However, Scripture speaks of punishment for sin not resulting from Adam's sin only, but also for our own sins. It is not merely our sin nature that condemns us but our actual transgressions (Romans 3:23). We did not eat the fruit, but we are certainly not without sin. At this point, you may want to object by stating that your sins are a result of being born with a sin nature. That is certainly true. However, before jumping to the conclusion that therefore God is unjust, remember this: God offers the free gift of salvation on the basis of His Son's perfect obedience and sacrificial death. How many of us complain that it is unjust that God should forgive us the sin-debt which was paid by Another (Colossians 2:13–14)? How many of us object that it is unfair that we should be reckoned righteous before God based on the obedience of Another (Romans 3:21–26)? The truly Innocent One dies so that guilty ones such as us may live. In a word, how many of us complain about God's grace being unjust? The gift of God's grace received through faith in Jesus Christ is greater than His judgment on Adam's sin and our own.
It is true that we did not eat the fruit, but every indication is that we would have sinned just as did Adam and Eve. So, yes, as their descendants we now carry the sin nature. We experience the consequences of living in a fallen world as well as the ill effects of our own sins. But we can also enjoy the mercy, grace, and forgiveness of God. As believers in Jesus, we reap the benefit of a righteousness that is not our own. He lived a righteous life and imputed His righteousness to us, taking our sin upon Himself so that we might be saved and made new (1 Corinthians 15:21–22; 2 Corinthians 5:17–21). We live in a world marred by sin now, but in Christ we are freed from the dominion of sin and one day we will be freed from the presence of sin (Romans 6; 8:29–30; 2 Corinthians 3:18; Philippians 1:6; Revelation 21—22 ).
Romans 5:15–21 explains, "But the free gift is not like the trespass. For if many died through one man's trespass, much more have the grace of God and the free gift by the grace of that one man Jesus Christ abounded for many. And the free gift is not like the result of that one man's sin. For the judgment following one trespass brought condemnation, but the free gift following many trespasses brought justification. For if, because of one man's trespass, death reigned through that one man, much more will those who receive the abundance of grace and the free gift of righteousness reign in life through the one man Jesus Christ. Therefore, as one trespass led to condemnation for all men, so one act of righteousness leads to justification and life for all men. For as by the one man's disobedience the many were made sinners, so by the one man's obedience the many will be made righteous. Now the law came in to increase the trespass, but where sin increased, grace abounded all the more, so that, as sin reigned in death, grace also might reign through righteousness leading to eternal life through Jesus Christ our Lord." "Thanks be to God for his inexpressible gift!" (2 Corinthians 9:15).
Does everyone inherit sin from Adam and Eve?
Original sin - What is it?
The sin nature - What is it?
The consequences of sin - What are they?
How can I be saved?
Truth about Sin