Are ideas about what is good ultimately only opinions?
The Golden Rule concerns treating people fairly, and treating people fairly is more than just an idea or opinion.
One reason people advocate for relativism, which is essentially the idea that good is subjective and opinionated, is because of the way good seems to take different forms across cultures. However, the underlying principles of good should not be confused with the ways in which they are manifested. For example, people bow in some places and shake hands in others, but the underlying principles of good behind a genuine greeting are the same.
People also can think good is subjective because good seems to change between situations within the same culture. However, the principles of good manifest themselves in different ways based on the context of a situation. A person can be angry at seeing a person mistreated, but that person shouldn’t get angry simply because someone annoyed them by mistake. It is not that it is subjective as to whether anger is good, but instead that good can manifest itself as anger when someone is being mistreated while manifesting itself as calm when someone mistakenly bothers us. Here, good is concerned with how another person should be treated in both cases.
The way other people should be treated is at the heart of the Golden Rule. The Golden Rule concerns treating people fairly, and treating people fairly is more than just an idea or opinion. In fact, it can be dangerous to think of good as only subjective or a matter of opinion because it can result in all kinds of inequality and other forms of mistreatment.