What's the difference between writing down a theme and a main idea?

A theme is like a statement about the message the writing was trying to get across, while a main point is more like a topic.

Explanation:

A theme will almost be a moral, and it is generally expressed in a sentence. A main idea is like a general thing that applies to the writing. You can build a theme statement off of a topic.

For example:

Topic/Main Idea: Friendship

Theme: Friendship is a powerful force

(Ik that is cheesy but its the best example I could come up with lol)

why does the crew join ahab's quest without hesitation?

what are you talking about