Of all the particular variations of relativism, I think my favorite might just be the historical version. A darling of progressive personages, this sentiment is commonly expressed in the form of "The Church must change to adapt to the needs of the 21st century", or when Sr. Joan Chittister, OSB said while covering the papal elections that "the very concept of sin is changing." [source] A major part of it is the belief that as man progresses and becomes more enlightened, our morality improves.
First to Sr. Chittister's definition of sin, then to the larger claim that our morality can progress. The basic definition of sin would be "a willful rejection of God's love". How this concept has changed at all since the beginning of time, I'm not quite sure.
The arguments against historical relativism are fairly simple: one, that the justifications for various actions have not changed significantly over the course of human history, and two, that the actions themselves have not changed significanlty over the course of human history. Same old sins, same old stories, basically. As Ecclesiates 1 says "What is it that hath been? The same thing that shall be. What is it that hath been done? The same that shall be done. Nothing under the sun is new." (9-10)
The part about the excuses I can attest to personally. I recall one occasion where a friend of mine was presenting an argument that she felt was a surefire lock for justifying homosexual behavior. She has put a lot of thought into it, and was convinced that she has a completely new approach to the matter. Interestingly enough, in trying to find a unique defense, she had inadvertently delivered an unusually similar argument as the Manicheean heresy back in the fourth century, namely, that sexual activities which could not result in procreation were to be preferred, since child birth was to be avoided. The key difference was the motive behind avoiding childbirth: one was concerned about overpopulation and doing everything possible to "reduce the problem", while Manicheeism held material existance to be evil, and that everything possible should be done to keep new material beings from existing.
In his book Just and Unjust Wars (which I would enthusiastically recommend to all people, regardless of interest in a military career), Michael Walzer, since he is of course primarily concerned with the military applications of ethics, wrote of common excuses given by soldiers charged with war crimes, obediance to orders from a superior and that they were caught up in the heat of the moment.
In Henry V, there is a scene where King Henry goes incognito with a cloak at night so that he is able to wander about the camp for a bit. Along the way, he meets three soldiers, and when they discuss whether the cause of the war is just, one of the soldiers says that they don't need to worry about it, "For we know enough, if we know we are the kings subjects: if his cause be wrong, our obedience to the king wipes the crime of it out of us."(IV:i:182-185) Compare that to the defense at Nuremburg, where the claim was that they were only obeying orders when they did what they did. 400 years have passed, but nothing seems to have changed. 300 years before Shakespeare, Aquinas also deals with the issue in II-II of his Summa, and from the sources that he cites, it is clear that even then he was not the first person to deal with this excuse.
If the excuses for a given action are the same over time, then that would tend to indicate that the action being excused are also pretty much the same. That and the fact that we haven't exactly had a great many new or improved thoughts in at least 2,000 years (and there's some who'd say that is a generous estimate). Were we to have some new moral code that is so dang superior to the old one, you'd almost expect us to behave differently than we have in years past, but we really don't.
If you read Augustine's Confessions, you see him pretty much run the gamut of sins, either from personal experience or refutation. Perhaps one of my favorite parts( though this isn't a sin per se) is towards the end of book X, when he describes a tendancy of ours which is essentially what we refer to as "rubbernecking". I laughed quite a bit when I read that, I must say. In addition, if you read Book I the Greater of Aristotle's Metaphysics, he goes over positions and opinions held by previous philosophers, you can definitely see the philosophies and opinions held by later philosophers. By the end of it all, you can find refutations for Hume, Kant, Descartes, and almost every major modern philosopher, and all this in the 4th century B.C.
This would be why I look with a cocked eyebrow whenever someone says that so-and-so has a brand-new idea that has revolutionized philosophy.
+Ad Majorem Dei Gloriam+
"The chief obstacle to the progress of the human race is the human race."