and also by participation in international consultative bodies. Three elements have been important in holding the communion together: first, the shared ecclesial structure of the component churches, manifested in an episcopal polity maintained through the apostolic succession of bishops and synodical government; second, the principle of belief expressed in worship, investing importance in approved prayer books and their rubrics; and third, the historical documents and the writings of early Anglican divines that have influenced the ethos of the communion. Originally, the Church of England was self-contained and relied for its unity and identity on its own history, its traditional legal and episcopal structure, and its status as an established church of the state. As such, Anglicanism was from the outset a movement with an explicitly episcopal polity, a characteristic that has been vital in maintaining the unity of the communion by conveying the episcopate's role in manifesting visible catholicity and ecumenism. Early in its development following the English Reformation, Anglicanism developed a vernacular prayer book, called the Book of Common Prayer. Unlike other traditions, Anglicanism has never been governed by a magisterium nor by appeal to one founding theologian, nor by an extra-credal summary of doctrine (such as the Westminster Confession of the Presbyterian churches). Instead, Anglicans have typically appealed to the Book of Common Prayer (1662) and its offshoots as a guide to Anglican theology and practise. This has had the effect of inculcating in Anglican identity and confession the principle of ("the law of praying [is] the law of believing"). Protracted conflict through the 17th century, with radical Protestants on the one hand and Roman Catholics who recognised the primacy of the Pope on the other, resulted in an association of churches that was both deliberately vague about doctrinal principles, yet bold in developing parameters of acceptable deviation. These parameters were most clearly articulated in the various rubrics of the successive prayer books, as well as the Thirty-nine Articles of Religion (1563). These articles have historically shaped and continue to direct the ethos of the communion, an ethos reinforced by its interpretation and expansion by such influential early theologians such as Richard Hooker, Lancelot Andrewes and John Cosin. With the expansion of the British Empire and the growth of Anglicanism outside Great Britain and Ireland, the communion sought to establish new vehicles of unity. The first major expressions of this were the Lambeth Conferences of the communion's bishops,