“A government which cannot protect its humblest citizens from outrage and injury is unworthy of the name and ought not to command the support of a free people.”

Second Inaugural Address (4 March 1893).

Statement to a reporter in the Boston Record, 14 April 1903. (quoted in Alpheus Thomas Mason, Brandeis: A Free Man's Life (1946), p. 122.)
Commonly paraphrased as "The most important office is that of the private citizen" or "The most important political office is that of the private citizen", and sometimes misattributed to his dissenting opinion in Olmstead v. United States.
Extra-judicial writings

Statement (26 June 1787) as quoted in Notes of the Secret Debates of the Federal Convention of 1787 http://avalon.law.yale.edu/18th_century/yates.asp by Robert Yates
1780s
Context: The man who is possessed of wealth, who lolls on his sofa or rolls in his carriage, cannot judge the wants or feelings of the day-laborer. The government we mean to erect is intended to last for ages. The landed interest, at present, is prevalent; but in process of time, when we approximate to the states and kingdoms of Europe, — when the number of landholders shall be comparatively small, through the various means of trade and manufactures, will not the landed interest be overbalanced in future elections, and unless wisely provided against, what will become of your government? In England, at this day, if elections were open to all classes of people, the property of landed proprietors would be insecure. An agrarian law would soon take place. If these observations be just, our government ought to secure the permanent interests of the country against innovation. Landholders ought to have a share in the government, to support these invaluable interests, and to balance and check the other. They ought to be so constituted as to protect the minority of the opulent against the majority. The senate, therefore, ought to be this body; and to answer these purposes, they ought to have permanency and stability.

Source: (1776), Book V, Chapter II, Part II, p. 892.

1920s, Equal Rights (1920)
Context: The doctrine of the Declaration of Independence predicated upon the glory of man and the corresponding duty to society that the rights of citizens ought to be protected with every power and resource of the state, and a government that does any less is false to the teachings of that great document — false to the name American. The assertion of human rights is naught but a call to human sacrifice. This is yet the spirit of the American people. Only so long as this flame burns shall we endure, and the light of liberty be shed over the nations of the earth. May the increase of the years increase for America only the devotion to this spirit, only the intensity of this flame, and the eternal truth of [Lowell's] lines: "What were our lives without thee, what all our lives to save thee, we reck not what we gave thee, we will not dare to doubt thee; but ask whatever else and we will dare".

International Journalism Festival http://www.journalismfestival.com/news/heather-brooke-antitrust-legislation-needed-to-keep-the-internet-free/ Interview with Fabio Chiusi, 12 April 2012.
Attributed, In the Media

Et ne savez-vous plus qu'il n'est princes ni rois
Qu'elle daigne égaler à ses moindres bourgeois?
Nicomède, act I, scene ii.
Nicomède (1651)

2010s, Free Speech and Its Present Crisis (2018)

Political Aphorisms, Moral and Philosophical Thoughts (1848)