Sin is one of those concepts that is universally understood, but when pressed for specifics, everyone's list is a little bit different. There are certain big sins that nearly everybody would agree on (murder, robbery, lying, etc.), but even those sins aren't always clear cut. Is it a sin to kill an attacker in self-defense, or an enemy soldier in time of war? Should a starving man who stole a loaf of bread be treated as a criminal? Is lying immoral even when telling the truth would cause greater harm? The world is a complex place, and gray areas abound.
To the fundamentalist, of course, there can be no gray areas. Either something is sinful, or it is not, and the Bible is believed to contain comprehensive instructions for telling the difference. With very rare exceptions, however, even fundamentalists don't follow every command found in the Bible; some amount of picking and choosing is conceded to be necessary to determine which sins are universal in nature and which were particular to ancient Israel and/or the first-century church.
The fact that no broad consensus exists on exactly which commands are still relevant today (aside from a few of the big ones) nonetheless fails to deter even some moderate evangelicals from making sweeping proclamations about "biblical morality." Likewise, the fact that the Bible fails to address many of today's pressing issues does little to deter those same believers from viewing it as a divine encyclopedia.
But as appealing as it may be to try to sweep away all of life's complexities by adhering to what is little more than a "soundbite morality" that can respond to every situation with a simple do or don't, such an approach is woefully inadequate to the realities of everyday life. What may be the best course of action in one situation may have disastrous consequences in another. What may help some people may be harmful to others.
Even medical science recognizes that no two people are exactly alike, as doctors and researchers increasingly move away from talking about "the" cure for a disease in favor of addressing the cure for Joe's disease, which may differ in important ways from the cure for Sally's disease, even if Joe and Sally were both given the same basic diagnosis. Similarly, our legal system has historically understood that breaking the letter of the law does not necessarily mean an individual is deserving of punishment; each case is unique, hence the presence of a jury composed of ordinary citizens who have the power to acquit a defendant even if the facts of the case clearly point toward guilt.
How, then, can we ever determine what is morally right if we have to take into account the uniqueness and complexity of each individual? The short answer is that we can't, at least not as absolutely as the fundamentalists in every belief system insist that we must be able to. And fortunately, the Bible doesn't require us to. By boiling everything down to two essential commands (love God and love others), we are free to pursue that which is good without having to forcibly twist every situation to fit a rigid, one-size-fits-all mold.
In addition, we have the Holy Spirit as a guide to help us determine what is right in any given circumstance. Fundamentalists don't care much for the individualized nature of the Holy Spirit's guidance; within their legalistic mindset, only an iron-fisted authority that treats everyone exactly the same can possibly be just, and so the Spirit is relegated to Divine law enforcement duty or pushed into the background entirely.
Back to the issue at hand, based on our two essential commands we can place all sins into two basic categories: sins against others and sins against God. Both categories are broad enough to be ambiguous, but the former is the more tangible of the two. When we cause harm to another person, whether through action or inaction, malice or neglect, we sin against them.
Even then, it's not always clear-cut; when the needs of multiple people conflict, it's not always possible to prevent harm to everybody involved. Sometimes a lesser harm is necessary to prevent a greater harm. And even acts of genuine compassion can go awry. The assistance that helps one person get back on his feet may hinder the recovery of another by creating dependency. The confrontation that corrects one person's course may emotionally destroy another. The advice that mends one relationship may permanently ruin another.
I'm convinced that much of the success the early church lay in the fact that it was composed of small, close-knit communities. The only way we can truly love people - to make the world a better place for them and minimize our sins against them - is to get to know them intimately. Likewise, even when we know a person very well we don't always know what's best for them, no matter what the Bible appears to say on the subject. The less time we have invested in getting to know someone, the less valuable our advice to them can be.
Even under ideal circumstances we can't always know in advance what the outcome of our actions will be. Part of adulthood is learning how to make reasonably educated choices, and learning how to accept responsibility for the consequences of those choices.
It is, in fact, responsibility that fundamentalists fear most. For them, the consequence of being wrong is eternal punishment, so they necessarily formulate a theology that enables them to divest themselves of all responsibility. If one's actions are done out of obedience to what "the Bible says," then any negative consequences cannot truly be bad since God is responsible for them. The detachment from reality that this mindset creates becomes validation that one is becoming more "heavenly minded," and thus it feeds upon itself.
But if the sins we commit against others cannot be measured in terms of real-world consequences, then the command "love your neighbor as yourself" is useless at best, and dangerously misleading at worst. If the spiritual effects of our actions are so detached from their physical and emotional effects that the latter must sometimes be disregarded as Satanic deception, then there is no limit to the abuses that we can commit in God's name once we've invented a "biblical" support for them.
Similarly, we must be careful about what we proclaim to be a sin against God. We can certainly learn from the Bible some of the things that distress God, but if human beings are too complex for us to neatly compartmentalize, how much more beyond our grasp is an infinite being? And God, who understands us better than we understand ourselves, is going to be less caught up in the superficiality of outward appearances than we mortals are. One person's action that appears flippant from our vantage point may in fact be an act of sincere devotion. Only God Himself is wise enough to discern the difference, and we run the risk of sinning against both him and our brother or sister when we are too quick to condemn that which we don't fully understand.
Some might argue for a third category of sin: sins against oneself, or any action that might be considered a "victimless crime." Sometimes self-harm is arguably a sin against those around us, given how interconnected all but the most isolated individuals are, and there's a case to be made for considering it a sin against God in any case, since we are harming somebody that He loves.
Even here, however, life is too complex to divide into neat categories. Just because somebody is overweight, for example, it doesn't automatically follow that they are a glutton. As in all other things, at the end of the day the best we can do is to humbly acknowledge how little we actually know as we actively seek to learn God's direction for our own lives - a daunting enough task without throwing in the burden of trying to determine God's will for anyone else.
Setting aside our need to be in control of everything can be extremely difficult to do - fundamentalists, for all their talk about submitting to God's authority, merely seek to hold onto that control by reducing God's will to a cudgel that can be wielded against others (violently if necessary) in the name of love. Real love for others, however, respects the uniqueness and autonomy of others by relinquishing our desire to reshape them in our own image. And real love for God includes giving up every effort to usurp the authority that can only rightfully belong to him.