The indoctrination game is a complete-information contest over public opinion. The players exert costly effort to manifest their private opinions in public in order to control the discussion, so that the governing opinion is similar to theirs. Our analysis provides a theoretical foundation for the silent majority and vocal minority phenomena, i.e., we show that all moderate opinions remain mute in equilibrium while allowing extremists full control of the discussion. Moreover, we prove that elevated exposure to others' opinions increases the observed polarization among individuals. Using these results, we formulate a new social-learning framework, referred to as an indoctrination process.