Statesboro, GA, USA
When it comes to the question of what is and is not sin ... there are of course grey areas, and marginal places, where it seems we have to determine whether or not something is right or wrong mainly based upon our consciences and experiential knowledge. But the question of those things are usually settled in the light of some other moral principle we are more sure of. So though they are somewhat less clear, that doesn't mean they are totally unclear. I'm sure there is honest disagreement among moral people about certain things. And there is also the possibility of self-deception, and thinking evil to be good, all the while possessing some knowledge to the contrary. We're not always able to determine whether a person is practicing legitimate determination from the other, but God does.
But despite areas of difference, there is a striking trans-cultural, trans-religious uniformity in moral standards. It's not absolutely the same, but the foundations are essentially the same. I think that's why the Bible states that God wrote his laws "on their hearts", referring even to those peoples / nations who did not have the benefit of recieving the embodiment of God's will as expressed in the Mosaic Law.
When Jesus came, he did not introduce a really new ethical/moral system. He may have amplified the signal, but the signal was the same. What he did for the Jews was show them that it was impossible to meet the demands of God by meeting the demands of the law, and suggest another way. Then he went on to take their deficit and failure in his own body on the cross. He turned men from law to love, and brought the grace (unmerited favor) of God to light. That much was original .. but the moral system wasn't. Nor was the moral system of the Jews all that original, though it may have been clearer than the moral codes of the Pagans, having come more directly from Heaven. But the moral systems (expressed through law and custom) of the Gentiles were not really unlike that of the Hebrews.
If you're ever interested check out the appendix of C.S. Lewis' "The Abolition of Man". He does a harmonization of moral principles expressed in various moral/ legal codes of ancient civilizations (Babylon, Egypt, Greece, Rome etc...). Their similarities are more notable than any differences in my opinion.
Lastly, I think your own conscience provides somewhat of an answer. It think in every stage of my life (whatever religious persuasion I was at the moment) I knew there was such a thing as a real right and wrong when it came to behavior and thought. I bet the same is true for you. Can you tell me all the guilt you feel is "Catholic Guilt"? Even your moral reproof of the abuses of an overbearing priesthood of the RC church, tells me that you think them really wrong in some of their actions. Even your offense has a moralistic base.
I'm aware that you could easily tell me that's just a creed. You did put that clause "apart from creed" in your question. But my response would be that a creed is nothing but a dogmatized description of something more real, more vivid, and more actual than itself. As G.K. Chesterton wrote, the doctrine of Original sin is "practical as potatoes", and the most empirical part of the cardinal Christian beliefs.