This study is the first to put together and analyze Plato’s two-part presentation of Protagoras, the most famous sophist of all time. In the Protagoras Plato sets out the sophist’s moral-political teaching, and in the Theaetetus we learn of his theoretical doctrine. Protagoras turns out to be a devout atheist contemptuous of all ordinary morality, and he claims on that basis to teach his best students what wisdom is. Going together with this harsh moral-political teaching is a daunting theoretical one, according to which “a human being is the measure of all things.” In Protagoras’ hands this amounts to a radical relativism: how things appear to each really are for each, there being no stable, “objective” world against which to measure our necessarily private perceptions. Plato reveals that Protagoras was led to adopt that relativism in order to respond to the threat or challenge posed by religious piety to the very possibility of philosophy, understood as the way of life guided by autonomous human reason. Socrates too was concerned with that challenge, but in his engagement with Protagoras it becomes clear that he neither denigrated ordinary moral life nor succumbed to the temptation of a radical relativism.