The monumental Rotunda in Thessaloniki was originally part of the palace complex of the emperor Galerius, who so famously persecuted Christians in the early fourth century. It was converted to a Christian church by the late fourth or early fifth century, a conversion which included the addition of a magnificent mosaic program within the dome of the Rotunda. This article both addresses the question of what this conversion meant and seeks to articulate a method for interpreting the Rotunda's archaeological remains within a local context, broadly construed. Texts produced in Thessaloniki or which were known to be objects of civic pride, such as 1 Thessalonians, an account of local fourth-century martyrdoms, and the triumphal Arch of Galerius, are used to interpret the Rotunda's conversion. This article concludes that the production and early interpretations and use of the early Christian Rotunda are revolutionary in that Christians reused monumental space built by a Roman emperor who persecuted Christians. Even more, a revolution in meaning occurred because the early Christian Rotunda can be interpreted as having borrowed from an apocalyptic rhetoric that formerly subverted empire, turning this around in order to articulate a Christian Roman identity continuous with and supportive of the Roman Empire.