system
A chat on subreddit /r/NoStupidQuestions
human
This depends on who you ask and how you define mythology. In the academic sense, mythology is folklore narrative that often deals with gods and creation. The word does not necessarily imply that subject matter is false or not, it simply means that it is a story of sorts. In this case Christianity contains a lot of mythology. The story of Adam and Eve is a myth. Again, in the academic sense, that does not necessarily mean it is wrong, but rather a story of sorts.

Religion is myth combined with philosophy/theology. As I mentioned before, Christianity contains a lot of myths, but it also contains instructions on how to live. This is not a narrative like a myth, but a set of rules and beliefs. It may be told through a myth, but the message exists without the myth. The story of how Moses brought the tablets down from the mountain is myth, but Ten Commandants themselves are philosophy. When you have philosophy supported by myth, you have a religion.

The Greek beliefs are not quite as philosophical, which is why some academics do not call it a religion. They have a lot of myths, but what morals and orders do those myths force us to live by? Sure, Zeus loves to have sex with a bunch of women, but what does that do for humans? Because of this lack of instruction in life, it may not be a full religion. Of course, there was some instruction and some certainly lived in a way according to the myths (the Eleusinian Mysteries comes to mind), but it does not seem as integral as the philosophy in Christianity.

The non-academic use of the word basically means "something that is not real". If I said to the common person that Adam and Eve were a myth, they would interpret that as not being real. In that case, the Greek beliefs are called myths mainly because we assume they are wrong, and call Christianity religion because we assume it is right. This use is even somewhat present in academia nowadays which leads to more people calling it the Greek religion to avoid implying that it is inferior to the Christianity and other beliefs we would call religions.
gpt
This was an amazing explanation, thank you so much!