“The day may dawn when fair play, love for one's fellow men, respect for justice and freedom, will enable tormented generations to march forth triumphant from the hideous epoch in which we have to dwell. Meanwhile, never flinch, never weary, never despair.”

Address to the annual meeting of the Phoenix Chamber of Commerce (30 March 1961)
Later variant: Freedom is a fragile thing and is never more than one generation away from extinction. It is not ours by inheritance; it must be fought for and defended constantly by each generation, for it comes only once to a people. Those who have known freedom and then lost it have never known it again.
California Gubernatorial Inauguration Speech http://www.reagan.utexas.edu/archives/speeches/govspeech/01051967a.htm (5 January 1967)
1960s
Context: Freedom is never more than one generation away from extinction. We didn’t pass it on to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same, or one day we will spend our sunset years telling our children and our children's children what it was once like in the United States where men were free.

Award Winner's Speech http://www.leipzig-award.org/englisch/rede_preistraeger_2003.htm, Leipzig Human Rights Award, 2003 Leipzig Human Rights Award of the European-American Citizens Committee for Human Rights and Religious Freedom in the USA, (May 18, 2003).

Source: What I Saw At Shiloh (1881), V

A Thanksgiving Sermon (1897)
Context: I thank the great scientists—those who have reached the foundation, the bed-rock—who have built upon facts—the great scientists, in whose presence theologians look silly and feel malicious. The scientists never persecuted, never imprisoned their fellow-men. They forged no chains, built no dungeons, erected no scaffolds—tore no flesh with red hot pincers—dislocated no joints on racks—crushed no bones in iron boots—extinguished no eyes—tore out no tongues and lighted no fagots. They did not pretend to be inspired—did not claim to be prophets or saints or to have been born again. They were only intelligent and honest men. They did not appeal to force or fear. They did not regard men as slaves to be ruled by torture, by lash and chain, nor as children to be cheated with illusions, rocked in the cradle of an idiot creed and soothed by a lullaby of lies. They did not wound—they healed. They did not kill—they lengthened life. They did not enslave—they broke the chains and made men free. They sowed the seeds of knowledge, and many millions have reaped, are reaping, and will reap the harvest of joy.

Source: The Greatest Salesman in the World (1968), Ch. 14 : The Scroll Marked VII, p. 86.

Utopia and Violence (1947)

1963, Third State of the Union Address

The Sense of Wonder (1965)
Context: Those who dwell, as scientists or laymen, among the beauties and mysteries of the earth are never alone or weary of life. Whatever the vexations or concerns of their personal lives, their thoughts can find paths that lead to inner contentment and to renewed excitement in living. Those who contemplate the beauty of the earth find reserves of strength that will endure as long as life lasts.