Epideixis –Demonstration of the apostolic preaching– is a brief and almost unknown work written by St. Irenaeus of Lyon, of which an Armenian manuscript was found in 1904. This work presents the procedures followed by the apostles when announcing the mystery of Christ by means of validation evidences found in the Old Testament. In this article we pay attention to the concrete study of the use of the Pentateuch made by the author in the first part of his book. Besides the analysis of the whole set of texts, their selection, way of use (additions, omissions, and changes when contrasting to the biblical texts), and the apocryphal references are also analyzed. Finally, a brief reflection about a possible original contribution from Irenaeus to the
teachings of the Holy Fathers is presented.