“[T]he most important position in a democracy is not the office of the President. The most important office is the office of citizen, because if you have citizens who are informed and know about other countries, and recognize that if we provide foreign aid to some distant country in Africa, that ultimately may make us healthier. And if you have a citizenry that recognizes that even if I have to pay slightly more in taxes — which nobody likes paying taxes -- but if I do, maybe I can provide that young child who lives in a poorer neighborhood an opportunity for a better life. And then because she has a job and a better life, she can pay taxes, and then everybody has more, and the society is better off. If you don't have citizens like that, then you're going to get leaders who think very narrowly and you'll be disappointed. So the job — one thing I always tell young people, don't just think that you elect somebody and then you expect them to solve all your problems and then you just sit back and complain when it doesn't happen. You have to work as a citizen also to provide the leaders the space and the direction to do the right thing. It's just as important for you to challenge ignorance or discrimination or people who are always thinking in terms of war”

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

2017, Farewell Address (January 2017)
Context: It falls to each of us to be those anxious, jealous guardians of our democracy; to embrace the joyous task we've been given to continually try to improve this great nation of ours. Because for all our outward differences, we, in fact, all share the same proud title, the most important office in a democracy: Citizen. Citizen.

2010s, Interview with Bill Kristol (2016)

Quoted from his book https://en.wikipedia.org/wiki/Success_and_Failure_Based_on_Reason_and_Reality, "Success and Failure Based on Reason and Reality" https://www.amazon.co.uk/SUCCESS-FAILURE-BASED-REASON-REALITY/dp/9970983903/
2018

This isn't some giveaway to people who are on welfare. This is giving help to people who are working hard every day.
Remarks at a a rally in Lake Worth, Florida (21 October 2008) http://www-tc.pbs.org/newshour/rss/media/2008/10/21/20081021_wrap.mp3
2008

Closing address to the National Youth Peace camp at Debua, 9 December 2005.

2008, 2008 Republican National Convention

Source: 1880s, Incidents and Anecdotes of the Civil War (1885), pp. 202– 203

Quoted in Johnny We Hardly Knew Ye, Kenneth O'Donnell, Dave Powers, and Joseph McCarthy, Boston: Little, Brown, and Company, 1970, page 262.
Attributed