Irenicism in Christian theology refers to attempts to unify Christian apologetical systems by using reason as an essential attribute. The word derives from the Greek word ειρήνη (eirene) meaning peace. It is a concept related to natural theology, and opposed to polemicism or war-like argumentation, and rooted in the ideals of pacifism. Those who affiliate themselves with irenicism identify the importance of unity in the Christian church, and declare the common bond between all Christians under Christ.
Erasmus and his influence 
Desiderius Erasmus was a Christian humanist and reformer, in the sense of checking clerical abuses, honoring inner piety, considering reason as meaningful in theology as in other ways. He also promoted the notion that Christianity must remain under one church, both theologically and literally; under the body of the Catholic Church. Since his time, irenicism has postulated removing conflicts between different Christian creeds by way of mediation and gradual amalgamation of theological differences. Erasmus wrote extensively on topics related generally to peace, and an irenic approach is part of the texture of his thought, both on theology and in relation to politics:
Despite the frequency and severity of polemics directed against him, Erasmus continued ... to practice a kind of discourse that is critical and ironic, yet modest and irenic.
Certain important irenic contributions from Erasmus helped to further the humanist consideration of themes of peace religious conciliation; these included the Inquisitio de fide (1524), arguing against the papal opinion that Martin Luther was a heretic, and De sarcienda ecclesiae concordia (1533). Erasmus had close associates sharing his views (Julius von Pflug, Christoph von Stadion, and Jakob Ziegler), and was followed on the Catholic side by George Cassander and Georg Witzel.
The influence of Erasmus was, however, limited, by the virtual exclusion of his works from countries such as France, from 1525, at least in the open; though they did appear in numerous forms and translations. James Hutton speaks of "the surreptitious manner in which Erasmus' peace propaganda reached the French public."
Franciscus Junius published in 1593 Le paisible Chrestien arguing for religious tolerance. He addressed Philip II of Spain, using arguments taken from the French politique statesman Michel de l'Hôpital and reformer Sebastian Castellio.
17th century: Catholics and Protestants 
Irenic movements were influential in the 17th century, and irenicism, for example in the form of Gottfried Leibniz's efforts to reunite Catholics and Protestants, is in some ways a forerunner to the more modern ecumenical movements.
The 1589 Examen pacifique de la doctrine des Huguenots by Henry Constable proved influential, for example on Christopher Potter and William Forbes. Richard Montagu admired Cassander and Andreas Fricius. The 1628 Syllabus aliquot synodorum was a bibliography of the literature of religious concord, compiled by Jean Hotman, Marquis de Villers-St-Paul decades earlier, and seen into print by Hugo Grotius using the pseudonym "Theodosius Irenaeus," with a preface by Matthias Bernegger.
It was typical enough, however, for moderate and even irenical writers on the Catholic side to find in this period that their arguments were turned back against Catholicism. This style of arguing developed in England from Thomas Bell and particularly Thomas Morton. It led to Thomas James mining Marcantonio de Dominis and Paolo Sarpi, and making efforts to claim Witzel for the Protestant tradition; to the arguments of Gallicanism being welcomed but also treated as particularly insidious; and an irenist such as Francis a Sancta Clara being attacked strongly by firm Calvinists. The handful of Protestant writers who were convinced in their irenic approach to Catholics included William Covell and Thomas Dove.
17th century: Protestant divisions 
James I of England thought that the Bible translation he commissioned might effect some reconciliation between the English Protestant religious factions, and prove an irenicon. The Greek ἐιρηνικόν (eirenikon) or peace proposal is also seen as irenicum in its Latin version.
An irenic literature developed, relating to divisions within Protestantism, particularly in the twenty years after the Peace of Westphalia. Examples marked out by title are:
- David Pareus, Irenicum sive de unione et synodo Evangelicorum (1614)
- John Forbes, Irenicum Amatoribus Veritatis et Pacis in Ecclesia Scotiana (Aberdeen, 1629)
- Jeremiah Burroughs, Irenicum (1653)
- John Dury, Irenicum: in quo casus conscientiæ inter ecclesias evangelicas pacis, breviter proponuntur & decidunter (1654)
- Daniel Zwicker, Irenicum irenicorum (1658)
- Edward Stillingfleet, Irenicum: A Weapon Salve for the Church's Wounds (1659 and 1661)
- Matthew Newcomen. Irenicum; or, An essay towards a brotherly peace & union, between those of the congregational and presbyterian way (1659)
- Moses Amyraut, Irenicum sive de ratione pacis in religionis negotio inter Evangelicos (1662)
- Samuel Mather, Irenicum: or an Essay for Union (1680)
Evaluation of early modern irenicism 
Anthony Milton writes:
[Ecumenical historians] have tended to assume the existence of an irenical 'essentialism' in which the association of Christian unity with peace, toleration and ecumenism is presupposed. [...] In fact, most thinkers of this period accepted that religious unity was a good idea, in the same way that they believed that sin was a bad idea. The problem was that, of course, different people wanted irenicism on different terms. [...] Different interpretations of irenicism could have direct political implications, making the rhetoric of Christian unity an important tool in the political conflicts of the period.
It is in that light that he comments on the irenists' succession: Erasmus, Cassander, Jacob Acontius, Grotius, then John Dury, who spent much time on a proposed reconciliation of Lutherans and Calvinists.
Modern usage 
Irenical has become a commonly-used adjective to design an idealist and pacific conception, such as the democratic peace theory.
False irenicism or false eirenism is an expression used in certain 20th-century documents of the Catholic Church to criticize attempts at ecumenism that would allow Catholic doctrine to be distorted or clouded. Documents using the term include the encyclical Humani Generis, promulgated by Pope Pius XII in 1950, and the Second Vatican Council's 1964 Decree on Ecumenism, Unitatis Redintegratio.
- Terence J. Martin, Living Words: Studies in Dialogues about Religion (1998), p. 278.
- Peter G. Bietenholz, Thomas Brian Deutscher, Contemporaries of Erasmus: A Biographical Register of the Renaissance and Reformation (2003), p. 78, p. 275 and p. 475.
- Nick Thompson, The Long Reach of Reformation Irenicism: the Considerationes Modestae et Pacificae of William Forbes (1585-1634). pp. 124-147 in Reforming the Reformation (2004); PDF (at p. 2 and p. 8).
- James Hutton, Themes of Peace in Renaissance Poetry (1984), p. 146.
- Martin van Gelderen, The Political Thought of the Dutch Revolt 1555-1590 (2002), p. 83; Google Books.
- Anthony Milton, Catholic and Reformed: The Roman and Protestant Churches in English Protestant Thought, 1600-1640 (2002), pp. 248–9; Google Books.
- W. B. Patterson, King James VI and I and the Reunion of Christendom (2000), p. 149; Google Books.
- Anthony Milton, Catholic and Reformed: The Roman and Protestant Churches in English Protestant Thought, 1600-1640 (2002), pp. 233-9.
- Nicolson, Adam (2003). God's Secretaries. p. 66.
- James E. Force, Richard Henry Popkin, Newton and Religion: Context, Nature, and Influence (1999), p. 146 and p. 175.
- Anthony Milton, ’The Unchanged Peacemaker’? John Dury and the politics of Irenicism in England, 1628-1643, p. 96 in Mark Greengrass (editor), Samuel Hartlib and Universal Reformation: Studies in Intellectual Communication (2002).
Further reading 
- Howard Louthan (1997), The Quest for Compromise: Peacemakers in Counter-Reformation Vienna
- Joris van Eijnatten (1998), Mutua Christianorum Tolerantia: Irenicism and Toleration in the Netherlands: The Stinstra Affair, 1740-1745
- Samuel J. T. Miller, Molanus, Lutheran Irenicist (1633–1722) Church History, Vol. 22, No. 3 (Sep., 1953), pp. 197–218
- Bodo Nischan, John Bergius: Irenicism and the Beginnings of Official Religious Toleration in Brandenburg-Prussia, Church History, vol. 51 (1982), pp. 389–404
- Michael B. Lukens, Witzel and Erasmian Irenicism in the 1530s, The Journal of Theological Studies 1988 39(1):134-136
- Graeme Murdock,The Boundaries of Reformed Irenicism: Hungary and Transylvania in Howard Louthan, Randall Zachman (eds), From Conciliarism to Confessional Church, 1400-1618 (South Bend: Notre Dame Press, 2004).
- Daphne M. Wedgbury, Protestant Irenicism and the Millennium: Mede and the Hartlib Circle, in Jeffrey K. Jue (editor), Heaven Upon Earth: Joseph Mede (1586–1638) and the Legacy of Millenarianism (2006)