The Umayyad caliph Marwan ibn al-Hakam (r. 64-65/684-685) is the subject of great controversy in Islamic narrative sources. There is general agreement on the basic facts of his life and career, but--as with so many other figures of the first half-century of Islam--his character and, in particular, his legitimacy as caliph have been debated. Some reports in the narrative sources emphasize Marwan's knowledge of religious law (fiqh) and the Qur'an, while many others denigrate him as a libertine or as the architect of an oppressive and essentially "un-Islamic" regime. The figure of Marwan is thus enmeshed in intense polemic (particularly between the Umayyad family and the family of 'Ali and their respective backers), and on the basis of the narrative sources it is difficult for the modern researcher to decide where the truth lies in the flurry of charges and counter-charges, idealizations and vilifications. We are not quite completely at the mercy of the narrative sources, however; although we lack almost entirely any true documents dealing with Marwan that could shed a more sober light on his character and beliefs, the genealogical information provided about Marwan's family offers some important clues on these matters. The paper analyzes this onomastic information to show that, in fact, Marwan seems to have been an early, ardent champion of what we now call Islam. Although he was born and raised in the more ecumenical monotheistic "Believers' movement" begun by Muhammad, Marwan seems to have been one of the first to emphasize the centrality of the Qur'an and the figure of the prophet Muhammad as hallmarks of the community to which he belonged. He thus played a central role in generating the gradual emergence, from the matrix of the Believers' movement, of Islam as a distinct religious confession, marked most fundamentally by this dual focus on the Qur'an and Muhammad as prophet.