Abstract: While most studies of Messianic Jews focus on how they grapple with anxieties of in-authenticity in relation to the broader Jewish community, this article considers how adherents understand their faith as a unique form of authenticity. On one level, both Messianic Jewish claims of authenticity and critics of Messianic authenticity reflect the same cultural logic of what I call the “evaluative grammar of authenticity.” The evaluative grammar of authenticity values causal/metonymic indexes over manipulated symbols and is undergirded by a suspicion that general appearances are symbolically manipulated in order to mask actual indexical underpinnings. This article argues that the strong stance on Messianic Jewish authenticity in this community is facilitated by the employment of the evaluative grammar of authenticity within a model of reality strongly influenced by the eschatology and epistemology of American Christian fundamentalism. The indexical underpinnings of the cosmos within this model of reality make it logical to conceive of the Messianic Jewish movement as a manifestation of authentic biblical religion. This mode of authenticity is briefly compared to that reflected by critics of Messianic Jewish authenticity who tend to employ this evaluative grammar within a more natural/historical model of reality. This ethnographic example is useful for exploring some of the basic contours of conflicts over authenticity, including how the value-laden domains of knowledge and agency are implicated in these conflicts. It also illustrates how the evaluative grammar of authenticity exemplifies a shared cultural value that, due to its internal logic, tends to engender division and cultural heterogeneity as much, or more, than it engenders cultural consensus.