This article investigates the discourse behaviour of Latin unus (‘one’) in the Vulgate, in order to better understand the starting point of the grammaticalization process ultimately leading to an indefinite article. To this end, I will study unity cardinals in general, focusing particularly on English one. Empirically, it will be shown that unus behaved like a unity cardinal in late Latin, without showing any clear sign of ongoing grammaticalization towards an indefinite article. I will try to explain the discursive behaviour of unity cardinals with reference to antipresuppositions, within the framework of the game of same and different. Finally, I will sketch how the results fit within the general picture of the incipient grammaticalization of indefinite articles, by providing an account of so far unexplained scope phenomena.