The Catholic Church remained the dominant form of Western Christianity in Britain throughout the Middle Ages, but the (Anglican) Church of England became the independent established church in England and Wales in 1534 as a result of the English Reformation.
In Scotland, the Presbyterian Church of Scotland, established in a separate Scottish Reformation in the sixteenth century, is recognized as the national church.
Scholars have suggested multiple possible reasons for the decline, but have not agreed on their relative importance.
Martin Wellings lays out the "classical model" of secularisation, while noting that it has been challenged by some scholars.
The United Kingdom was formed by the union of previously independent countries in 1707, and consequently most of the largest religious groups do not have UK-wide organisational structures.
While some groups have separate structures for the individual countries of the United Kingdom, others have a single structure covering England and Wales or Great Britain.
An Ipsos MORI survey fielded in August 2003 indicated that 18% of respondents claimed to be "a practising member of an organised religion" and 25% claimed "I am a non-practising member of an organised religion".
Forms of Christianity have dominated religious life in what is now the United Kingdom for over 1,400 years.
It was introduced by the Romans to what is now England, Wales, and Southern Scotland.
Among Christians, Anglicans are the most common denomination, followed by the Catholics, Presbyterians, Methodists and Baptists.
This, and the relatively large number of individuals with nominal or no religious affiliations, has led commentators to variously describe the United Kingdom as a multi-faith and secularised society.