Irenaeus, as bishop of Lugdunum (modern Lyon in France), wrote his five- volume work Against Heresies to counteract the spread of Gnostic ideas within the Christian Church. Gnostic Christians claimed to possess higher “knowledge” (gnosis, in Greek) based on secret teachings that Jesus kept from ordinary believers and reserved only for an elite group. By the middle of the second century, Gnostic sects were multiplying and varied widely in doctrine and practice, as each teacher proposed a different private revelation of knowledge. Against Heresies reflects this diversity, as Irenaeus takes on each sect in turn and refutes the doctrines of its founder or current teachers. Gnosticism represented one of the earliest major doctrinal challenges within Christianity, and it was difficult to oppose because it was polymorphous, changing constantly depending on the idiosyncrasies of its teachers.