The text plays on the double meaning of the word 'speechless'. In one sense, it refers to the gods being unable to speak because the theists (their believers) are not speaking on their behalf. In another sense, it humorously suggests that the gods themselves would literally have nothing to say if the theists stopped talking, as if the gods' ability to speak is dependent on the theists.
If the theists all shut up , the gods would be speechless .