All things being equal, those who have more power are liable to sin more; no theorem in geometry is more certain than this.

[Caeteris paribus, on trouvera tousjours que ceux qui ont plus de puissance sont sujets à pécher davantage; et il n’y a point de théorème de géométrie qui soit plus asseuré que cette proposition.]

Gottfried Wilhelm Leibniz (1646-1716) German mathematician, philosopher, diplomat, polymath
Letter to Ernst, Landgrave of Hessen-Rheinfels (9 Jul 1688) [tr. Fasnacht (1952)]
    (Source)

Quoted by John Dalberg, Lord Acton (and thus often attributed to him).

Acton's quotation was in his Inaugural Lecture on History, Cambridge (11 Jun 1895). In the lecture, after mentioning the academic precept "never be surprised by the crumbling of an idol or the disclosure of a skeleton; judge talent at its best and character at its worst; suspect power more than vice," he footnotes this Leibniz quotation (in its source French, with the Latin introduction). This was in turn translated into English in G. E. Fasnacht, Acton's Political Philosophy, ch. 6 (1952), after which it became erroneously cited by others to Acton.

The source letter (in which Leibniz is discussing the Jesuits) is collected in Gottfried Wilhelm Leibniz, Sämtliche Schriften und Briefe, Series 2, vol. 2, p. 278 (2009), reprinted in Stephen Voss, The Leibniz Arnauld Correspondence (2016) (the Source noted), which offers this alternate translation:

Other things being equal, one will always find that those who have more power are subject to sin more. And there is no theorem of Geometry more sure than this proposition.
[tr. Voss (2016)]