Catholicism is a broad term for the body of the Catholic faith, its theologies and doctrines, its liturgical, ethical, spiritual, and behavioral characteristics, as well as a religious people as a whole.
For many the term usually refers to Christians and churches belonging to the Roman Catholic Church in full communion with the Holy See. For others it refers to the churches of the first millennium, including, besides the Roman Catholic Church, the Eastern Orthodox Church, the Oriental Orthodox Church, and the Assyrian Church of the East. And for others again it refers to "adhering to the catholic faith as it has been inherited from the earliest Christians ... seeking to restore the faith and order of the primitive church", as claimed by the Anglican Communion  and the Reformation and post-Reformation churches.
The Catholic claim of continuity is based on, among other factors, Apostolic succession, especially in conjunction with adherence to the Nicene Creed.
In the sense of indicating historical continuity of faith and practice, the term "catholicism" is at times employed to mark a contrast to Protestantism, which tends to look solely to the Bible as interpreted on the principles of the 16th-century Protestant Reformation as its ultimate standard. It was thus used by the Oxford Movement.
This contrast, however, is not so clearcut for Anglican theologians such as Alister McGrath and others[who?]. According to McGrath, "Anglicanism is not a 'middle way' between Protestantism and Catholicism. For that reason ... it is neither Protestant nor Catholic, but combines the strengths of both. Yet historians such as Diarmaid McCulloch [an Anglican] have claimed that the 'middle way' developed in England in the late 16th century was between Lutheranism and Calvinism—two quite distinct versions of Protestantism. The 'middle way' which resulted was neither Calvinist nor Lutheran—but it was certainly Protestant." ...