Ernst Peter Wilhelm Troeltsch (German: [tʁœltʃ]; 17 February 1865, Haunstetten – 1 February 1923, Berlin) was a German Protestant theologian and writer on philosophy of religion and philosophy of history, and an influential figure in German thought before 1914, including as a member of the history of religions school. His work was a synthesis of a number of strands, drawing on Albrecht Ritschl, Max Weber's conception of sociology, and the Neo-Kantians of the Baden school.
His The Social Teachings of the Christian Church (two volume edition in translation by Harper Row, 1960) is a seminal work in this area.
Troeltsch was born into a Lutheran family to a doctor but went to a Catholic school in a predominantly Catholic area before attending university in Erlangen and then Göttingen. His ordination in 1889 was followed in 1891 by a post teaching theology at Göttingen. In 1892, he moved on to teach at the University of Bonn; in 1894, he moved on again to Heidelberg University. Finally, in 1915, he transferred to teach at what is now the Humboldt University of Berlin where he took the title of professor of philosophy and civilization.
Throughout Troeltsch's life, he wrote frequently of his belief that changes in society posed a threat to Christian religion, that "the disenchantment of the world" as described by sociologist Max Weber was under way. At an academic conference that took place in 1896, following a paper on the doctrine of logos, Troeltsch responded by saying "Gentlemen, everything is tottering!"
Troeltsch sought to explain the decline of religion in the modern era through a description of the historical evolution of the role of religion in society. He described European civilisation as having three periods: ancient, medieval and modern. Troeltsch's understanding of the border between the medieval and modern periods is revisionary: instead of claiming that modernity starts with the rise of Protestantism, Troeltsch argues that early Protestantism should be understood as a continuation of the medieval period. The modern period starts much later on his account. It was later, in the seventeenth century, that the modern period truly began. The Renaissance in Italy and the scientific revolution planted the seeds for the arrival of the modern period, and Protestantism delayed rather than heralded its onset. Protestantism, Troeltsch argued, was "in the first place, simply a modification of Catholicism, in which the Catholic formulation of the problems was retained, while a different answer was given to them".
Troeltsch saw this distinction between early and late (or "neo-") Protestantism as "the presupposition for any historical understanding of Protestantism".
Troeltsch developed three principles pertaining to critical historical inquiry. Each of the principles served as a philosophical retort for the issue of the preconceived notions sustained by the historian. Troeltsch's three principles - the principle of criticism, the principle of analogy, and the principle of correlation - were determined to account for the issue surrounding the biases of the historian. 
Principle of Criticism
Troeltsch's claim in this principle concludes that absolutes within history cannot exist. Troeltsch surmised that judgments about the past must be varied. In such, the absolute truth of historical reality could not exist, rather, he claimed historical situation could only be examined as more likely or less likely to have happened. In this, Troeltsch understood to never create a finite and non-revisable claim.
Principle of Analogy
This principle pertains to averting the historian from applying anachronism to the past. Troeltsch understands that the probability in the former principle can only be validated if a historians present situation, when assessing the probability, is not radically different from the past. In this, Troeltsch expects that human nature has been fairly constant throughout time, however this clause is still included as a form of accountability for the historian's narrative.
Principle of Correlation
In regards to a historical events, Troeltsch determined, through this principle, humanity's historical life is interdependent upon each individual. This understanding applies a casual nature to all events, equaling thus, an effect. Any radical event, the historian should assume, effected the historical nexus immediately surrounding such and event. Troeltsch determines that in historical explanation it is important to include antecedents and consequences of events. This is in an effort to maintain historical events in their conditioned time and space.
Troeltsch was politically liberal and served as a member of the Parliament of the Grand Duchy of Baden. In 1918, he joined the German Democratic Party (DDP). He strongly supported Germany's role in World War I: "Yesterday we took up arms. Listen to the ethos that resounds in the splendour of heroism: To your weapons, To your weapons!"
In the immediate aftermath of Troeltsch's death, his work was considered passé and irrelevant. This was part of a wider rejection of liberal thought with the rise of Neo-Orthodoxy in Protestant theology, especially with the prominence of Karl Barth in the German speaking world. From 1960 onwards though, Troeltsch's thought has seen a revival of interest in academic circles with a variety of books being published on Troeltsch's theological and sociological work.
- Michael Walsh (ed.). Dictionary of Christian Biography. Continuum. p. 1108. ISBN 0826452639.
- Robert J. Rubanowice (1982). Crisis in consciousness: The Thought of Ernst Troeltsch. Tallahassee: University Presses of Florida. p. 9. ISBN 0813007216.
- Quoted in Rubanowice, p. 21, from Protestantism and Progress (translated by W. Montgomery, 1958), p. 59. Original title: Die Bedeutung des Protestantismus für die Entstehung de modernen Welt.
- Quoted in Toshimasa Yasukata (1986). Ernst Troeltsch: Systematic Theologian of Radical Historicality. Atlanta, Georgia: Scholars Press. p. 51. ISBN 1555400698.
- Harvey, Van Austin (1966). The Historian and The Believer: The Morality of Historical Knowledge and Christian Faith. New York: Macmillan Company. pp. 13–15.
- Troeltsch, Ernst. Gesammelte Schriften II. pp. 729–753.
- Emma Wallis (9 April 2014). "Ernst Troeltsch and the power of the pen". DW.de.
- Garrett E. Paul. "Why Troeltsch? Why today? Theology for the 21st Century". Religion-Online.
- Gerrish, B. A. (1975). Jesus, Myth, and History: Troeltsch's Stand in the "Christ-Myth" Debate. The Journal of Religion 55 (1): 13-35.
- Pauck, Wilhelm. Harnack and Troeltsch: Two historical theologians (Wipf and Stock Publishers, 2015)
- Nix, Jr., Echol, Ernst Troeltsch and Comparative Theology (Peter Lang Publishing; 2010) 247 pages; a study of Troeltsch and the contemporary American philosopher and theologian Robert Neville (b. 1939).
- Troesltch, Ernst, "Historiography" in James Hastings (ed.), Encyclopedia of Religion and Ethics (New York: Charles Scribner's Sons, 1914), VI, 716-723.
- Bill Fraatz. "Ernst Troeltsch (1865-1923)". Boston Collaborative Encyclopedia of Western Theology.
- G. Cantillo, Introduzione a Troeltsch,"I Filosofi", Laterza, Roma-Bari 2004.