It is often taught that the human race was brought into condemnation because, somehow, everyone is involved in Adam's first sin which he committed by eating the fruit that God had forbidden him to eat. The result is that the whole human race, is under the condemnation of God.
For those who are not familiar with the Bible and Christian ideas, I should explain that Adam was the first person created by God. A little later God took a rib from Adam and used it to make the first woman who Adam called Eve. God forbade them to eat the fruit from the “tree of knowledge” but Adam and Eve disobeyed.
These are some objections to this doctrine and answers to those objections:
Objection 1: How can God justly hold us responsible for what Adam did?
The quick answer to this is that he doesn't. We are not blamed for Adam's sin and Adam's sins are not held to our account. We are condemned because we are sinners. What we need to grasp is that Adam's character has put us in that place where we can justly be condemned by God.  Let me try and explain with an example:
Imagine that a man and a woman are living in a nice safe country with lots of green plants, cuddly animals and four big rivers, far away from anything even remotely dangerous. In the middle of the land there is a vast rift valley, several miles wide and many miles long, and the sides of the rift are sheer rock faces several hundred metres high and quite unclimbable. Now one day the man and the woman build a couple of parachutes and jump over the edge of the cliff. Well, they land with a bit of a bump, but without any serious injuries and they make a new home down in the valley. The valley is very nice but one thing they hadn't realised was that rift valleys are places of volcanic activity and, in the case of this particular valley, the next eruption is going to turn the whole valley floor into bubbling, red hot, liquid rock. Oops! Anyway, before long the woman is pregnant and in due time gives birth to their first child, and then, they have a few more. Well time goes by and the man and the woman die but by then the valley is full of their descendants. One day many, many years later one of these descendant develops an interest in seismology and discovers that an eruption is due to occur within a just few weeks or months and, because they cannot get out of the valley, that they are all about to perish. As the news is disseminated, there begins a great outcry; everybody wants to know why they should all suffer this nasty end and they are told it is because their great, great grandparents jumped from a place of safety. But, protest the people, we didn't jump from a place of safety so why should we suffer for what they did? Well, they are told, when our ancestors jumped we came with them in their genes. When we were born we were born here because that is where they were. It is not the people who jumped who must suffer, but the people in the valley. If they hadn't jumped we would have been born somewhere else. As it is we are now living in this place of great danger and here we must perish — unless we are rescued.
In the same way, Adam's sinful nature has consequences for everybody descended from him and since everybody is descended from him, the consequences apply to us all. Adam moved into that spiritual and moral territory called “Sin” and his descendants were therefore born “in Sin”. Everybody born “in Sin” is a sinner; God rightly condemns sinners. Or, to look at this another way, Adam's sinful nature has put us in a place where we cannot avoid sinning ourselves. Either way, we are condemned because of our own sinful nature and our own sin.
Objection 2: I don't understand how the character of Adam or the consequences of Adam's disobedience can be transmitted through the generations.
Well, frankly nor do I. But then I don't understand how my body uses oxygen or how my brain controls my lungs (in fact for the first few years of my life I never even knew I had a brain or lungs) but I have always known that it was better to keep on breathing. It is not very important whether or not we understand the exact mechanism by which Adam's sin involves us; but when Adam disobeyed God, something happened in the legal, moral and spiritual history of our race such that all descendants of Adam and Eve are born “in [the territory we call] sin” and are, therefore, under God's righteous wrath. Because Adam's nature was imperfect, every human ever born is incapable of always doing right and therefore every human will sin.
Objection 3: Little children do not know what sin is.
It is popularly believed that children are “innocent”. We need to be careful how we use this word and know what shade of meaning we are giving it. It is true that children are not generally responsible for the major tragedies of the world — they have not started wars and they do not usually have criminally oriented minds. However although children may not be officially guilty of a legal crime against society that does not mean that they have never sinned: Remember that as far as God is concerned, murder is no worse than lying and lying is no worse than selfishness because any and all sins are sufficient to cut us off from God. Some people might ask how a baby still unable to speak could possibly have sinned. To answer this we need to ask how they can be so sure that the baby has NOT sinned. The sins of a heart are not always obvious in an adult but they would be impossible to detect in a baby that has only a very limited ability to communicate.
It is worth examining how a sin is expressed: There is a difference between knowing or committing sin and being able to express sin. A baby too young to talk is incapable of verbally expressing any form of rebellion but that does not mean that it is not self centred, envious, or just plain evil... Must we then claim that every expression of need by a child is a selfish, sinful demand? No, we are only saying that the baby has no means of expression except by the physical use of its body (making faces, crying, waving arms around etc.). This is an especially limited form of expression for a child because children are often physically restrained and constrained by their clothes, cot, chair, and other things that they cannot easily overcome.
Now this very restricted form of expression is sufficient for making simple statements such as “I want attention”, “I want food”, “I'm uncomfortable”; but it is not adequate for making the complex statements needed to explain “why”. The infant has no means to communicate its reasons and motives — only its conclusions. This inability to communicate reasons and motives does not mean that there are no reasons or motives but it does mean that if and where they exist, we must be, and must remain, ignorant of them.
Now if we have no way of detecting a baby's sin then we are not in a position to say that the baby has no sin. The very obvious retort to this is that if we cannot automatically declare the baby innocent then neither can we automatically declare the baby guilty because there is no evidence visible to us by which we can decide either way. This is a fair response but in fact we can know that the child is a sinner by working backwards through a person's life and asking “When does sin begin?” If children are born pure, at what point do they become sinful? If they only become sinful when they do something which is morally wrong, what then is their reason for doing the wrong thing? Who, being pure, would desire what is not right? If we try and explain their wrong action as a misunderstanding, lack of knowledge or inexperience then we are saying that sin is merely social or cultural phenomena that can be removed by education. If this were true then once these things were explained to the child then it, being pure, should no longer want to do them. Furthermore, if sin is a manifestation of ignorance then God is not just; if sin is only mistakes made in ignorance, then what is the grounds for condemnation? Since when has ignorance been a judicial or legal problem? Justice does not condemn the ignorant but the guilty; those who have deliberately done what they already knew to be wrong. If we believe in a holy God then we must also believe that sin is evil because God condemns it. If we believe that sin is the result of only a cultural, social or educational problem then we have no right to punish the guilty. Finally, If we try and say that the sin only happened when the sinful desire arose then we must explain how the desire to do something wrong could exist in a pure person. The fact that the desire exists proves that they are not pure!
So, when does the transition from purity to corruption occur? The painful truth is that it doesn't:— Because we know that we are sinners now, and because it is not possible to explain how a pure person can sin or desire to sin, we therefore know that we can never have been pure; as the Bible says, we were born and conceived in sin.
So we conclude that we do not become sinners but rather our very nature is that of a sinner. Sin is what sinners do. Sin (which is the doing of what is morally wrong) is the natural act of a sinful character. We are not sinners because we sin, we sin because we are sinners.  The sinful nature is what we have first, the sin follows naturally behind. In the same way a slug did not become a slug because it left a slime trail behind it but, rather, it leaves a slime trail because it is a slug. If it had never gone anywhere and therefore had never left a slime trail, it would still be a slug. A person commits sins because he or she is a sinner. It is not only sins that are excluded from the presence of God, but also the people whose nature is sinful — and that includes you, me and everyone.
The sinful nature and other faiths
The concept of a sinful nature seems to run contrary to what most people would like to believe. The world in general prefers to think that people are born good and only become corrupt as they get older; this is the official teaching of Islam and the Quran. According to popular opinion, children are “innocent”. How anybody could believe this, I confess, I cannot understand. I would have thought that a cursory but honest reflection on one's own childhood was bound to convince anybody that children are as guilty as adults but perhaps I am the only person who was, as a child, disobedient, selfish, spiteful, vindictive and generally nasty for as far back as I can remember (which is back to when I was roughly three years old). I like to think that I have improved since then!
Relevant Bible verses:
...sin came into the world through one man...  Paul's letter to the Christians in Rome 5:12 (Amp)
I have been evil from the day I was born; from the time I was conceived I have been sinful.  Psalm 51:5 (TEV)
All of us were like sheep that were lost, each of us going his own way.  Isaiah 53:6 (TEV)
The Lord looks down from heaven at human beings to see if there are any who are wise, any who worship him.  But they have all gone wrong, they are all equally bad.  Not one of them does what is right, not a single one.  Psalm 14:2-3 (TEV)
...  for all have sinned and fallen short of the glory of God. Paul's letter to the Christians in Rome 3:23 (NIV)
Jesus replied, "The most important [commandment] is this: `Listen, Israel.  The Lord our God is the only Lord.  Love the Lord your God with all your heart, with all your soul, with all your mind and with all your strength'".  The good news as told by Mark, 12:29 (TEV)
Whoever breaks one commandment is guilty of breaking them all.  The letter of James 2:10 (TEV)
"...  But Christ has indeed been raised from the dead, the first fruits of those who have fallen asleep. For since death came through a man, the resurrection of the dead comes also through a man. For as in Adam all die, so in Christ all will be made alive. But each in his own turn: Christ, the firstfruits; then, when he comes, those who belong to him. Then the end will come, when he hands over the kingdom to God the Father after he has destroyed all dominion, authority and power. For he must reign until he has put all his enemies under his feet. The last enemy to be destroyed is death. For he "has put everything under his feet". Now when it says that "everything" has been put under him, it is clear that this does not include God himself, who put everything under Christ. When he has done this, then the Son himself will be made subject to him who put everything under him, so that God may be all in all." Paul's first letter to the Christians in Corinth 15:20-28 NIV