One last try.

Dave,
Steve,
be ready to be challenged to support your opinion, to demonstrate that
you are not prejudiced, bigoted or otherwise unduly partial. Even if
you're not challenged, you should be honest enough to offer that
support in any case. That would mark you out as a person of integrity,
not a mere charlatan.

I agree Steve, and Richard and others made the same point.
It is the point I made to Dave that he so objected to, and led him to
start this thread.

I said,
"An opinion based on no actual knowledge is just a whim from an empty
head and deserves to be dismissed.
Please share with us the knowledge that your opinion is based on."

(He had expressed the view that mainstream media and BBC reporting was      
a major cause of radicalisation to join IS)