The question was put: "What is sin and what isn't?"
To which I responded:
In my view, sin consists of any of the following broad categories:
(1) Going against your conscience, when your conscience is correct. This of course raises an infinite number of further questions, foremost of which is how can we reliably determine that one's conscience is correct? I am a strong believer in the principle that the conscience can err, and that these errors are often traceable to the prejudices we have gained from our upbringing, experience, and even self interest. There is no single sure-fire way to guaranty that our conscience will be error free. I personally believe that we as children of God have a sizeable chunk of our conscience which springs from the divinity in us, i.e., what in Mormonism has been termed the "Light of Christ". But our conciensce (i.e., our internal sense of right and wrong) is heavily influenced by other things as well, not all of which stem from Christ. Although we cannot ever make our consciences perfect in this life, we can make them less prone to error by studying, pondering, and reasoning. The "when your conscience is correct" condition is also critical. The mere fact that we believe we are sinning because we feel guilty does not necessarily mean we are in fact sinning. For conservative mormons, a good example might be a teenager who grows up in a strongly anti-Mormon evangelical household, and who feels guilty for exploring Mormonism because, among other things, he knows his parents do not approve.
(2) Choosing, unreasonably, to remain ignorant on issues which bear on theology, morality, happiness, freedom, human progress, etc. This is in many ways related to the first category. Because our ability to strive for perfection and happiness depends so heavily upon our conscience, we have a moral duty to educate that conscience so that our internal compass of right and wrong can be better honed. Willful ignorance is, in my own opinion, among the greatest of sins.
(3) Refusing to seriously and fairly consider at least the possibility of the truth of various matters without having objective and irrefutable proof. People should not be blamed for ultimately choosing not to believe things which are unverifiable through either their own human experience or the scientific method. But I do believe there are many truths which can only be discovered on an individual level through the exercise of faith, which entails a willingness to accept the possibility that truth exists which cannot be seen or objectively proven. Because I believe we have a moral duty to educate our conscience through the discovery of truth, I believe we have a duty to try out faith and see if it yields anything.
(4) Going against what your conscience SHOULD know to be right in light of objectively available evidence and arguments. I struggled with this one, because I believe that our culpability is much lower when we are actually ignorant of the truth. But at the same time, it seems right that there should exist standards of behavior and morality which are universal enough that people should be held to them even though their individual conscience has not ripened to the point that it has embraced or fully understood those standards. For example, I am not prepared to say that a psychopath is not sinning when he kills or tortures an innocent person, even though the psychopath's brain seems to lack the conscience to realize that what he is doing is very wrong. But I am also very wary of the slippery slope which could result from disagreements over what constitutes "objectively available evidence and arguments" or "standards of behavior and morality which are universal enough that people should be held to them". So to take an example for illustrative purposes, I don't think that a person is necessarily sinning if they don't follow the recommendation of President Thomas S. Monson to read the Book of Mormon daily. Stated another way, the perceived moral obligation to follow President Monson's counsel generally depends upon a highly subjective and far-from-universal sub-set of beliefs, over which reasonable minds can and do disagree. Beliefs which tend to be provable only or primarily by subjective spiritual experiences and theoligically nichey assumptions are not the "standards of behavior and morality which are universal enough that people should be held to them" which I am referring to. Generally, these universal standards will be secularly based and not rooted in theology.