This chapter discusses developments in the Christian rhetoric of free speech in the fourth century, after Christianity had become an accepted religion. A theological controversy arose, known as the Arian controversy, that pitched supporters of different interpretations of Christian doctrine against each other. Christians, who saw themselves as heirs of the martyrs, needed to find a new rhetoric of opposition that fitted the realities of the post-persecution era. Was it acceptable to inveigh against a Christian emperor because he subscribed to an alternative interpretation of Christ’s truth? This chapter focusses on the rhetoric of Bishop Hilary of Poitiers (d. 368) who came to be regarded as the head of the anti-Arian faction in the West and wrote an invective against Emperor Constantius II. In Hilary’s letters, we find a new vocabulary and rhetoric of free speech which covered a whole range from persuasion to criticism, from polite advice to outright abuse. The chapter shows how Hilary created a new, powerful image of a Christian truth-teller that was built on the cultivated memory of martyrs, apostles and prophets.