Don't argue with them unless you know what your talking about(like others have said) and don't get heated up either. Thats what they believe and understand and no amount of arguing is going to change that, but from the sounds of it you may not have much luck getting them to except your beliefs unless you put up a good arguement and stay calm and positive, if you get all aggresive like in your post they are only gonna blame satan.

I personally havn't believed in god for a few years . My parents never preached, infact my mother is an atheist.
Id keep your beliefs on the low for now, at least until you have an idea on how your parents will react. I take it they don't know you smoke weed?