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. The vast unexplored manuscript tradition of the Tiburtina has yielded a wealth of such evidence. 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. Scholars, especially textual critics, have ignored this possibility until now.