Theism, atheism, and agnosticism are each positions based on a network of assumptions. All 3 outlooks are plausible within their own presuppositional framework. Typically, the atheist or agnostic restricts herself to a scientific materistic framework of asumptions. Debate can be waged on whose assumptions are the most rational and explain all the data of reality in the most plausible manner. Since we believe our assumptions, atheism and agnosticism are by definition belief systems.
"On what grounds does the religious believer deny the existence of dragons?"
I suppose onn the grounds that he doesn't believe all the films and well-established eyewitness testimony about the Komodo Dragon of Indonesia. Sorry, I couldn't resist.
For the theist, the God postulate is falsifiable in principle, and yet, unfalsified. Santa Claus has sadly been falsified. But try telling that to little kids of parents in a Christmas eve service! The parents get incensed by such debunking!
"Rather, they would hold that an atheist is a person without a belief in God."
Supermodel, you seem to be fudging the distinction between "atheist" and "agnostic." An agnostic lacks a belief in God. She admits she doesn't know if there's a God. But God is irrelevant to her life and value system. By the way, I too would like to hear how you justify an afterlife with no appeal to an Intelligent Designer of that system.
Don