Like would they have taken the twelve labors of Hercules literally? Woud they believe that there was a man-bull hybrid under Crete? Did they think someone actually flew to close to the sun? Or would they say they are fictional events that feature the deities they believe in?
There are some great responses to your question in this thread. The top comment in particular by u/toldinstone provides great insight to the question of the degree to which the myths were believed.