A foundational work of early Christian theology, "Five Books of St. Irenaeus, Bishop of Lyons" is also referred to as "Against Heresies." The work, which was written in the second century by St. Irenaeus of Lyons, methodically disproves Gnosticism and other heresies of the day in order to uphold the traditional Christian faith. Irenaeus highlights the value of the Church's unity and apostolic heritage. He clarifies fundamental Christian beliefs and the persistence of the authentic teachings transmitted from the apostles through in-depth argumentation and textual allusions. This founding work is still crucial to comprehending the evolution of Christian theology and early Christian thinking.