Morality can easily be based upon the autonomous agency of rational creatures. The action of an autonomous being, when it impedes the ability of another autonomous being to self-govern, is the defining characteristic of an immoral act. This account is insensitive to culture or society, it is only sensitive to the nature of the interacting agents. It allows you to rule upon the rightness or otherwise of an act, regardless of whether that act is legal or not.
Now most people already recognize this account, especially when they are the offended agent.
You don’t need a divine figure, and you certainly don’t have to take the east way out into relativism (or indeed nihilism.)
- Sorry I don’t recall the original author
relativism
noun: the doctrine that knowledge, truth, and morality exist in relation to culture, society, or historical context, and are not absolute.
nihilism
noun: extreme skepticism maintaining that nothing in the world has a real existence.