God has clearly revealed his existence to men. Suppose a student were to write on a physics exam that he did not believe in atoms because he could not see them. Would not the professor be justified in failing him? The existence of atoms is clearly undeniable on the basis of their recognized effects. Everyone familiar with Hiroshima knows that atoms exist; they are known from their effect. Likewise, men are responsible to acknowledge God and his eternal power by the effects that are clearly revealed in the Creation.