This chapter considers the Sibylla Tiburtina's textual development in order to reinforce the argument that its popularity in the Middle Ages was informed by an interest in the Sibyl's theological role which was inherited from the Church Fathers. Textual variants of the Tiburtina are an invaluable resource for exploring its popularity in the medieval period: interpolations, deletions, and changes in the narrative structure give an insight into which aspects of the text were regarded as superfluous or important. Its christological content makes the Sibylline Gospel the obvious starting point to look for a connection between the Tiburtina's textual development and the traditionally christological deployment of Sibylline prophecy. This in turn means that not only the regnal list of the Ottonian Sibyl was recast in the eleventh century to bring it up to date, but also it's Sibylline Gospel. Isidore's Etymologiae circulated widely in the Middle Ages and was one of the key sources of information about the Sibyls.