Some seventy bishops of Irish birth and 150 of Irish descent are said to have attended the First Vatican Council in 1869–70. This reflected a great international phenomenon, the emigration of Irish Catholics to the United States and the four corners of the British empire in the century from 1815, in a river of people which became a flood during the Great Famine of 1846–9. The Irish ecclesiastical control of most of this emigration, outside Britain itself, was asserted by the Rome-trained and Rome-trusted Paul Cullen, archbishop of Armagh (1849–52) and then of Dublin (1852–78), apostolic delegate in Ireland with full authority from Rome, and from 1865 the first Irish cardinal. Cullen came to recommend the appointment of most of the bishops of the Irish diaspora, as it is now commonly known, and a stream of priests and religious also left Ireland to minister to their countrymen abroad, as the new territories slowly developed native Catholic institutions of their own.

This Irish empire of the spirit, compared by Catholics to the British Empire of the flesh, was rooted in the nineteenth-century Catholic Revival in Ireland itself, where despite the Penal Laws against the faith from the 1690s, four-fifths of the population had remained Catholic. The leading light of the revival was not a priest but the quintessential Irish Catholic layman, Daniel O’Connell (1775–1847), a utilitarian-minded lawyer, of Irish-speaking gentry stock, educated by English Catholics and at the English bar, and called the Liberator by his people. O’Connell reacted against the failure of the Irish rising of 1798 and against the union of the parliaments and established Protestant churches of England and Ireland to espouse the cause of a non-violent constitutional Catholic nationalism.

The nineteenth century was a period of paradox in the history of the Roman Catholic Church. It was an age of revival, especially through the growth of active religious orders of women, and the strengthening of Catholicism in areas in which it suffered from disadvantage or minority status – Germany, Holland, the British Isles and the English-speaking world–while coming under pressure or persecution in the traditionally Catholic Latin states of southern Europe and South America. Both renewal and decline had, however, a common strand, the new or neo-Ultramontane movement to exalt the authority of the pope as an inspiration for revival and a defence against decline among the ‘integrally’ religious. Neo-Ultramontanism was the trades unionism of priests resistant to bishops in France, the protection of bishops resistant to the state in Spain and Prussia, and the enthusiasm among converts from Protestantism to Catholicism resistant to various forms of unbelief in England, and among Catholics everywhere opposed to the seizure of the Papal States by the kingdom of Piedmont-Savoy between 1860 and 1870.

Neo-Ultramontanism upheld the traditionally Ultramontane doctrines of the infallibility of papal teaching in faith and morals and the universal jurisdiction of the Roman pontiff; it opposed the old Gallican theory, ascendant in the Catholic world before 1789, of an infallible authority restricted to general councils of the whole church, and of independent national churches effectively controlled by kings and states and acknowledging only a titular papal primacy. Gallicanism still prevailed in 1829, when, of the 646 diocesan bishops, the pope appointed only twenty-four outside the Papal States.

Historians of modern Christianity in western Europe, writing amid the chill winds of secularism at the beginning of the twenty-first century, may be tempted to apologise for their subject. Why write about something of diminishing importance, which has been in decline since the French Revolution? No student of the medieval or early modern eras doubts the central role of religion, but modern historiography can get along without it. In fact, the historian of nineteenth-century Christianity need not be defensive about his or her theme, which still entered into the very fabric of the social and political conflicts of the era, and just as the creation of a united Italy was on one level a defeat of Catholicism, and the creation of the German Empire a victory for Protestantism, so the attack upon the churches, in what some have seen as the beginning of secularisation, makes a fascinating story which, at least in the immediate term, led not only to religious decline but also to renewal and revival.

Western Europe might, however, be considered something of an anomaly even in the present, in which Christianity continues to grow and expand elsewhere, in the Third World, in the United States and, with the collapse of atheistic communism, in eastern Europe. This must be one reason for the somewhat unconventional appearance of this volume by the standards of other histories of the nineteenth-century Christian faith, as here at least a third of the space is given to the new Christian churches outside Europe. Catholic Christianity became a global religion through the Spanish and Portuguese empires in the sixteenth century and French missionaries in the seventeenth and eighteenth. There are chapters here reflecting the legacy of this earlier era.

Modern Ireland has appeared distinctive in two ways: first, in the complete identification of Irish nationalism with Irish Catholicism; and second, in the high levels of religious practice in Catholic Ireland until very recently, with over 90 per cent of the population attending mass every Sunday. Ireland has been unusual if not unique since the sixteenth century as a country in which the Counter-Reformation largely prevailed in the very teeth of persecution by a Protestant state, so that its Catholic faith was forged in the fire of suffering. Thus it has always been granted that in Ireland, at least from the time of Daniel O'Connell (1775–1847), Catholicism and Irish nationalism have reinforced each other. This relationship was sometimes troubled, and O'Connell himself drew the line against papal interventions in Irish politics by declaring ‘Our religion from Rome: our politics from home’, while the Irish Catholic nationalist movement was liberal in its sympathies in honouring its Protestant leaders, like the main protagonist of Home Rule for Ireland, Charles Stewart Parnell. Yet by drawing the priesthood into politics in the 1820s as the instruments of the first mass democratic movement in the modern world, the Catholic Association, O'Connell gave the clergy a perilous responsibility as the guardians of his pacific constitutionalist and democratic nationalism with its overt hostility to violence. Thus most churchmen suspected Irish nationalism in its revolutionary form, and they publicly denounced the main revolutions against British Protestant rule in Ireland in 1798, 1848 and 1867.