I agree with Little Lulu. It is odd that the prof would scold you for bringing up potentially controversial points of the book. Isn't that what discussions are for? Yeesh! How did other classmates feel about the discussion? I'm curious - what makes you think you lost credibility in the class? Credibility from whom? Sounds like the professor had an agenda for the class that day and made a fuss when the students veered "off-course" in his/her eyes.

Try not to be so hard on yourself. It was just one class, one day. I'm sure it won't matter so much to your classmates by next week. People forget stuff. Things blow over.