I don't think Theo in the dialogues is meant to be like an average believer; nor is Art meant to be like an average nonbeliever. Rather, the whole debate is there mostly as a hook on which to hang various lessons in how to address difficult and contentious topics. A real debater who spoke like Art would probably sound annoyingly pedantic and pompous (with the constant refrain of "now I am going to apply such-and-such a technique"), but of course those bits are really addressed to the reader much more than they are to Theo.