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The Old Testament is certainly the first sacred text for early Christians. Jewish Holy Book influenced the Gospel writers and early Christian authors thereby affecting the early Christian theological literature. The studies investigating the scale of this influence analysed how the Old Testament passages were quoted in the New Testament. As a result of these investigations, the nineteenth-century biblical scholars argued that in the first centuries of Christianity, the lists which were composed of the selected quotations from the Old Testament were used as a sacred text independently of the Bible. According to this, the first Christians quoted biblical passages specifically to support a belief and argument with the testimony of the Bible and attributed a forensic status to these quotations. In modern studies, these lists are called testimonia collections to underline the authoritative status of these collections and, even some of the researches argued that these collections existed before the Gospels. According to this claim, known as testimonia hypothesis, testimonia collections are the first sacred texts of Christianity. In this paper, we will explain the nature of the testimonia collections, related theories, and how these collections have been studied in Western biblical literature. In this study, we aim to present the history of the testimonia hypothesis and to provide a background for future studies. [You may find an extended abstract of this article after the bibliography.]