John Gill discusses the historical development and characteristics of Popery, emphasizing that it is not merely a title but a system of Antichristian doctrines and practices that have existed since the early church. He argues that the Pope of Rome is often viewed as Antichrist, and that the hierarchical structure and various doctrines associated with Popery have roots that predate the Reformation. Gill highlights the early emergence of practices such as the worship of saints, the prohibition of marriage, and the concept of purgatory, which he claims were already present in the church before the papacy reached its peak. He asserts that these elements are part of a broader mystery of iniquity that has infiltrated the church over centuries. The sermon serves as a warning against the acceptance of traditions that deviate from biblical teachings.