This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book examines the impact of emigration on the churches by exploring the consequences and potential consequences of mass population loss for each communion. It explores the theme of religious interpretations of the outflow by addressing a commonly referenced but only rarely scrutinised belief in emigration as a divinely dictated mission to spread Christianity across the globe, and consequent conceptions of an Irish 'spiritual empire'. The book relies for evidence on careful use of literary sources, the accounts of visitors to and travellers in Ireland, clerically authored pamphlets, parliamentary reports and manuscript material from religious archives. It surveys each of the Presbyterian, Anglican and Catholic churches practical religious involvement in the lives of emigrants, and in particular, the systematic provision of clergy by the home churches to emigrant communities.