“The arc of the moral universe may bend towards justice, but it doesn’t bend on its own. To secure the gains this country has made requires constant vigilance, not complacency. Whether by challenging those who erect new barriers to the vote, or ensuring that the scales of justice work equally for all, and the criminal justice system is not simply a pipeline from underfunded schools to overcrowded jails, it requires vigilance. And we'll suffer the occasional setback. But we will win these fights.”

1960s, Memorial Day speech (1963)

Address to the United Nations (1963)
Context: The Charter of the United Nations expresses the noblest aspirations of man: abjugation of force in the settlement of disputes between states; the assurance of human rights and fundamental freedoms for all without distinction as to race, sex, language or religion; the safeguarding of international peace and security.
But these, too, as were the phrases of the Covenant, are only words; their value depends wholly on our will to observe and honour them and give them content and meaning. The preservation of peace and the guaranteeing of man's basic freedoms and rights require courage and eternal vigilance: courage to speak and act — and if necessary, to suffer and die — for truth and justice; eternal vigilance, that the least transgression of international morality shall not go undetected and unremedied.
These lessons must be learned anew by each succeeding generation, and that generation is fortunate indeed which learns from other than its own bitter experience. This Organization and each of its members bear a crushing and awesome responsibility: to absorb the wisdom of history and to apply it to the problems of the present, in order that future generations may be born, and live, and die, in peace.

2011, Remarks on Egyptian political transition (February 2011)
Context: I know that a democratic Egypt can advance its role of responsible leadership not only in the region but around the world.
Egypt has played a pivotal role in human history for over 6,000 years.  But over the last few weeks, the wheel of history turned at a blinding pace as the Egyptian people demanded their universal rights.
We saw mothers and fathers carrying their children on their shoulders to show them what true freedom might look like.
We saw a young Egyptian say, “For the first time in my life, I really count.  My voice is heard.  Even though I’m only one person, this is the way real democracy works.”
We saw protesters chant “Selmiyya, selmiyya” — “We are peaceful” — again and again.
We saw a military that would not fire bullets at the people they were sworn to protect.
And we saw doctors and nurses rushing into the streets to care for those who were wounded, volunteers checking protesters to ensure that they were unarmed.
We saw people of faith praying together and chanting – “Muslims, Christians, We are one.”  And though we know that the strains between faiths still divide too many in this world and no single event will close that chasm immediately, these scenes remind us that we need not be defined by our differences.  We can be defined by the common humanity that we share.
And above all, we saw a new generation emerge — a generation that uses their own creativity and talent and technology to call for a government that represented their hopes and not their fears; a government that is responsive to their boundless aspirations.  One Egyptian put it simply:  Most people have discovered in the last few days…that they are worth something, and this cannot be taken away from them anymore, ever.
This is the power of human dignity, and it can never be denied. Egyptians have inspired us, and they’ve done so by putting the lie to the idea that justice is best gained through violence.  For in Egypt, it was the moral force of nonviolence — not terrorism, not mindless killing — but nonviolence, moral force that bent the arc of history toward justice once more.

This portion of Parker's sermon is thought to have inspired Martin Luther King, Jr.‎'s famous assertion of similar sentiments: "The arc of the moral universe is long, but it bends towards justice".
Ten Sermons of Religion (1853), III : Of Justice and the Conscience https://en.wikisource.org/wiki/Ten_Sermons_of_Religion/Of_Justice_and_the_Conscience

CIA probe 'not over' after Cheney's top aide indicted on CNN.com (October 28, 2005)

Trial of the Earl of Thanet, and others (1799), 27 How. St. Tr. 939.

Remarks by His Highness the Aga Khan at Évora University, Évora, Portugal (12 February 2006)]

2020s
Source: "The End of (Whig) History" https://www.nationalreview.com/2020/07/the-end-of-whig-history/?taid=5efd8dac17654f00015ab42c&utm_campaign=trueanthem&utm_medium=social&utm_source=twitter (1 July 2020), National Review