The Great Apostasy is a term used by some religious groups to describe a general fallen state of traditional Christianity, especially the Papacy, because they claim it allowed the traditional Roman mysteries and deities of solar monism such as Mithras and Sol Invictus and idol worship back into the church, and is not representative of the faith founded by Jesus and his twelve Apostles: in short, in their opinion, the church has fallen into apostasy. They feel that to conciliate the Pagans to nominal Christianity, the Catholic Church took measures to amalgamate the Christian and Pagan festivals so pagans would join the church; for example, bringing in the pagan festival of Easter as a substitute for the Pasch or Passover, although neither Jesus nor his Apostles enjoined the keeping of this or any other festival. It is officially agreed by the Catholic Church that it was magisterial Protestantism and Eastern Orthodoxy that fell into apostasy.
Read more about Great Apostasy: Overview, Protestant Views, Reformed Perspective, Traditionalist Catholic Perspective, Responses of Roman Catholicism and Eastern Orthodoxy