I've debated with scientists on other issues and they've said things like this to me. The flying spaghetti monster, purple unicorns and Santa Claus are favourites.
The key to this is that they are asking you to back up your position with evidence. It is, after all, what scientists have to do all the time. If you can't do it, then logically how is your particular claim or belief any more tenable than than any other claim or belief?
Also, please keep in mind that none of the posters here mean the Christian religion or God. It's the specific position some Christians happen to have, which claims that the earth is 6000 years old and that all life was created as it is today. There is a lot of evidence contrary to this belief, which is why you are being asked to explain what your own evidence is for your views. If you are claiming that creationism is scientific, then this is what you need to do. Is this what you are claiming?
How about some evidence for the Grand Canyon being less than 6000 years old, or carved by a flood?
