The comparison of the Irenaeus of Lyon s ecclesiology with the Lumen gentium arises in the context of the return to the Christian Sources in the nineteenth and twentieth centuries. This move had a capital influence in the Second Vatican Council concerning the process of aggiornamento of the Church in the modern world. In this regard, the fresh theological perspective, that was still growing, had an impact both in the ecclesiological discussions and the comprehension about the presence and action of the Church in the world. Furthermore, this whole set has increased in importance through the devising of the Dogmatic Constitution Lumen gentium. Particularly, Irenaeus, bishop of Lyon and martyr of the second century, emerges as a central source for the Lumen gentium since the Constitution includes five references of his writings, two of which are direct quotes. Both in Adversus haereses and Epideixis, the bishop of Lyon doesn t aim to write a theological treatise. Instead of that, his target is to defend the Christian faith, which was emancipating from the Jewish communities and achieving autonomy in the doctrine, the teachings, and the way of cult. In parallel with this emancipation process, different doctrines gain ground within Christianity, as, for example, the gnostic ones. However, according to Irenaeus, the only knowledge of the truth comes from the doctrine which the Church holds and teaches. As a result, for someone to belong to Christ it is mandatory to clearly know the regula fidei, which instructs about the comprehension of the Holy Scriptures, organizes the real God s cult, and guides the devotee to testify Christ s presence in the world. There isn t a systematic Theology about the Church in Irenaeus of Lyon‘s works, but this matter is recorded in many of his complaints and denials. It can be noticed by the fact that, to him, the Church is God s work. As the human being got apart from the Father s love, God has sent his own Son to recapitulate the whole creation and gather a New People, that could hear his voice and turn back towards him. Despite the ressourcement has applied new methodologies to build up theological thinking and to teach Theology, many Council Fathers were still used to an indirect theological approach, based on the use of handbooks. Therefore, comparing the Irenaeus ecclesiology to the one developed in Lumen gentium is an assignment that requires research both in the references of the conciliar text and the multiple conceptions of Church collected through analysis. Thus, it is possible to develop a deeper knowledge of a contemporary text in light of an ecclesiological model from the beginning of Christianity.