Hey TrueMyth:
Original sin just simply refers to the fact that all humanity has a fallen nature to do what is wrong rather than follow God's commandments. That is, we are predisposed to sin because it is in our sinful nature to do sin. The Bible records that we are sinners from birth (Psalm 51:5) and it is because we are sinners that we sin (i.e. our sinful nature predisposes us to want to sin).
I don't know how this sinful nature is "transmitted" but it seems to be in the soul of humanity; we can see the effects of original sin because we don't have to think about or try to do evil - it comes so easily and naturally, unlike being Christ-like. How much easier is it to bash some bloke who's been giving you trouble rather than treat him with love that Jesus taught?
If human beings are sinners from birth, then it is from birth that we deserve God's wrath, IMHO. I believe that we will only be held fully accountable and hence justly exposed to God's wrath when we can to a reasonable degree hear, understand, and comprehend God's offer of salvation and reject it anyway. I believe that babies who die will be in heaven, like-wise I believe that God will show mercy for those who have no heard or cannot comprehend the Gospel message (like the mentally disabled) as you said, how can you hold someone accountable for what they didn't know or couldn't comprehend. Granted, this doesn't completely excuse them other wise ignorance would be bliss. Paul instead says that there is enough evidence in the created world to point them to a knowledge of who God is (Romans 1:20). The Bible also says that God has placed eternity in the hearts of men (Ecclessiastics 3:11) so that they know there's more to life than here and now.
It is certain that human beings have a propensity to do evil. No right-thinking person can deny this. Further, it is not only that humanity as a whole has this difficulty, but each individual does bad things in their life. As the proverb goes, "To err is human." This is so clearly demonstrated through history and common sense that anyone who denies either of these propositions has their head irreparably in the clouds, in the land of gumdrops and lollipops.
From all of this, it is obvious that something about what it means to be human includes the certainty of evil. The question then becomes "What is the quality of this bad nature?"
The details are what concerns me. It is very simple (and very true) to say, "We are all sinners" or "Human beings are sinful". However, while this answers the question of why we see this feature in common human experience, it does not explain how this feature works. I want to find out. If I can't-- if this is one of those things I need to "be still and know"-- fine, I can deal with that. However, I want at least to clear up any contradictions in it so that I can prove to myself and others that Christian doctrine is at least not incoherent.
I honestly don't see how it is possible that we are sinful from birth. The common Christian formulation is that God is Holy; therefore, He cannot be around sin. If we are sinful at birth, then babies which die 2 hours after birth are sinful. Therefore, they cannot be around God; therefore, they go to Hell. This is a moral contradiction: If God is perfectly good and allows/causes/decrees this, then goodness is something we know not what, and we are as likely to find a Divine Fiend as a Divine Lover in heaven.
As for Psalm 51:5, remember that this is a Psalm-- roughly equivalent to a worship song today-- and thus may very well contain hyperbole, subjective expression, or outright falsehood. Remember the so-called "cursing Psalms"? Very un-Christian attitudes in those. We should not ignore or blow off any Scripture, but some of it is better for doctrine than others. What is clear from Psalm 51:5 is not that we are all ontologically sinful at birth, but that we often feel as if we were; in any case, we ought to feel that way in order to keep our pride at bay.
So if not at birth, when do we acquire this "sinful nature," whatever it might be like? Remember, sin is violation of God's Law. Where God's Law is not known, we have the Universal Moral Law, in our conscience. Every time we sin, we are aware of it. Thus, conscious awareness is required for sin to occur. When we are born, we slowly develop our sense of personhood and awareness of self as distinct from others. Our conscience and awareness of right and wrong also slowly coalesce. I doubt very much that until both of these are fully developed (and God knows when), we cannot be said to sin in our actions.
Earlier I spoke of babies and sin: your response to this is confusing, since you state both "It is from birth we deserve God's wrath" and "Babies who die will be in heaven". How can anything which deserves God's wrath be in heaven? If God is willing (to say nothing of able) to suspend His just wrath on a baby, why does He not do so for others who are deserving of it? This cuts to the heart of what is meant by "sinful human nature"-- Is it
this which brings God's wrath (and thereby hell) on us, or our sinful actions, freely chosen? You seem to acknewledge that some measure of conscious awareness needs to be present in order for a person to go to hell, but how can God's wrath and hell be separated? I'm not sure your position is logically consistent here, but I appreciate you offering it!
A sinful nature or being sinners doesn't make or force us to sin, it is a choice that we make. We are naturally inclined to sin (which is all what original sin and having a sinful nature means), but we are not forced to sin. This is what free will is - every time we need to we can make a conscious decision of how we want to react. Sometimes we speak before we think or we act before we think as I often do, but I still could have chosen to act godly rather than sin.
Aaah, this is the Million Dollar Question. As I said in other posts, I am all for the compatibilist freedom idea. We are inclined to sin (we desire to sin), and since we will always act according to our character (our inclinations/desires), we will sin unless God steps in and changes our inclinations/desires/character. This of course raises questions for those who do not accept Christ, however. If it is acknowledged that God must change our desires for us to choose Him, it is then proper to ask "Did God change the desires of those who do not choose Him?" If He did not, it is very hard to call Him good, let alone perfectly good. If He did, we must ask why they did not choose Him then. Did God not change them enough? Was there some other factor operating?
The only way out of this that I can see is to say that while all will certainly sin, and our desires are for evil, they are not always that way. To go back to the baby, I wonder if it is possible that our characters are not fixed in sin at that point. They are open to our choice of God or self. Thus, while we still only choose according to our desires/character, our desires/character are open to both options. The more choices we make, the more our desires/inclinations/character gets fixed. Since we live in a sinful world, and are raised by sinful parents, inevitably we will make the choice of self (sin). However, our hearts and our minds are still open to God, and He will continue to speak through whatever crack we allow Him until we finally shut the door in His face. Once shut, it will never open again. Remember: we will only choose according to our character. It is certain that all who are able to read the Bible are also able to realize that their character is tended toward evil. However, is it necessary to believe that it always is so? I don't think there is either good logical or Biblical support for this.
Therefore, to go back to the question of those who do not choose God, I think it is probable that God begins speaking to them from a very early age, working against the effect of sin in the world to keep their hearts open to Him. The more times the person chooses God, the more open they will be to His further advances. The more times the person chooses self, the more closed they will be to His further advances. God alone knows when His intervention would override their free will; He will not cross that line. As the person freely chooses to further fix their character either for or against God, He will continue to give grace to choose Him, until finally their character is fixed either completely surrendered to Him (heaven) or rebelling against Him (hell).
Sin, to be blunt, is missing the mark of God's perfect standards. It is rejecting God's way and truth in favour of our own. This is exactly what Adam and Eve did when they ate of the forbidden fruit ("how great it would be to become wise... [and become like God]").
At least, this post represents my basic understanding of it, which may change in the future as I learn more. Hence, I'm not too sure how accurate it is.
I may change mine too-- I have no illusions that I have it all right! I do, however, want to eliminate contradiction and suggest options for solutions.