The Latin phrase lex orandi, lex credendi ('the law of praying is the law of believing') is a phrase which is often used in Anglican theological discussion, but which needs careful unpacking if its meaning is to be properly understood. In this study Martin Davie provides such unpacking. He traces the history of the phrase back to its origins in the work of St. Prosper of Aquitaine in the fifth century, explains what it means and gives examples of how it has been both used and misused in the Roman Catholic, Orthodox and Anglican traditions. His conclusion is that when it is rightly understood the principle lex orandi, lex credendi provides a useful tool for assessing both a church's liturgy and its doctrine. It reminds us that a church's liturgical practice needs to cohere with its doctrine and both need to be in line with Scripture. He also argues that the use of this tool shows us that not only are proposals for marking same-sex relationships unacceptable, but so also is the new proposal to use liturgy to mark gender transition. Martin Davie is a theological consultant for the Church of England Evangelical Council and the Oxford Centre for Religion and Public Life. He also teaches doctrine at Wycliffe Hall, Oxford