Is the definition of "sin" subjective? Of whom's morals is it based on?

Whenever I try to determine what is a sin and what isn't I get very confused... I read things by Paul and sins seem to become very subjective... \_\_\_\_\_ is a sin if you make it a sin through belief that it isn't acceptable due to your obeying of the law (the old law, before Jesus came) then I read the old testament which makes it much more objective... \_\_\_\_\_ is a sin because it is immoral (Referring to God's morals) If the first is true than couldn't anything be justified as not-sin simply by a choice in a state of mind and what you believe your morals are (if that's the case why would you need Jesus for salvation, you are already free of "sin"... so that *obviously* isn't the case) If the second is true than how do we ever know what is sin and what isn't? (The spirit that is given to us.) Okay, but what about before the spirit came (personally and historically), how could anyone interpret wether or not they are sinning? I know the law was given but surely the law couldn't cover everything? Is it all just a matter of the heart? I read about "sexual immorality" but what is that even referring to, immoral things in your conscience and the culture you live in or immoral as a definition of a standard of God that was never given to man... I see people who look at everything sexual as a biological process so of course they never feel guilt or remorse and it would seem almost unjust to classify their actions as sin. I see others who see it as something where \_\_\_\_ is culturally acceptable but \_\_\_\_ is not and is therefor sin. How does one define sin? _I know it seems like a broad question but I can't simplify it any further than what I've typed above._