A search of the Hebrew manuscripts and incunabula in the Vatican Library and other large Judaica collections yielded many dictionaries and citation indexes but almost no subject indexes, although the latter are common in Latin Christian manuscripts and incunabula. Reasons for this are suggested, and the compilation of early indexes for religious purposes is discussed. General conclusions relate to the methodological and terminological difficulties of conducting research on the earliest indexes, which were compiled in languages and scripts that are little known. The original manuscripts of early indexes are scattered in libraries throughout the world. Writings on the history of indexing are not concentrated in a single discipline: Descriptions of early dictionaries, concordances, and indexes are often found in religious publications rather than the literature of library-information science. The earliest reference tools related to religion are presented in a chronological table.