What is faith? It usually means religious belief, and sometimes diverse religions are grouped together as "faiths," with reference to "faith leaders" or "faith schools." What we have faith in matters, and that involves our reason, involving claims to truths that affect everyone. Faith is not just a personal attribute, like tastes, but should be open to public examination and debate. In this Very Short Introduction, Roger Trigg argues that all faith needs reason. He puts contemporary discussions into historical perspective, particularly in the context of Christianity. The author argues that faith also involves a commitment to action and that matters for all social life. Because religion is typically directed at what is seen as of crucial importance for human life, faith should not be marginalized or privatized. It will permeate every idea of how people should behave, and has a role in the public square, however respectful it should be to competing views within a democracy.