Behaving on a still higher moral level were the astronauts who went to the Moon, for their actions tend toward the survival of the entire race of mankind. The door they opened leads to the hope that H. sapiens will survive indefinitely long, even longer than this solid planet on which we stand tonight. As a direct result of what they did, it is now possible that the human race will never die.
Many short-sighted fools think that going to the Moon was just a stunt. But the astronauts knew the meaning of what they were doing, as is shown by Neil Armstrong's first words in stepping down onto the soil of Luna: "One small step for a man, one giant leap for mankind."
The Pragmatics of Patriotism (1973)

“Marriage is not ownership and wives are not property.”

Robert A. Heinlein book The Puppet Masters

Source: The Puppet Masters (1951), Chapter 21 (p. 116)

“No philosophy that he had ever heard or read gave any reasonable purpose for man's existence, nor any rational clue to his proper conduct. Basking in the sunshine might be as good a thing to do with one's life as any other — but it was not for him and he knew it, even if he could not define how he knew it.”

Robert A. Heinlein book Methuselah's Children

Methuselah's Children (1958)

“Jim?”
“Yes, Dad.”
“What’s this about leaving your gun where the baby could reach it?”
Jim flushed. “It wasn’t charged, Dad.”
“If all the people who had been killed with unloaded guns were laid end to end it would make quite a line up. You are proud of being a licensed gun wearer, aren’t you?
“Uh, yes, sir.”
“And I’m proud to have you be one. It means you are a responsible, trusted adult. But when I sponsored you before the Council and stood up with you when you took your oath, I guaranteed that you would obey the regulations and follow the code, wholeheartedly and all the time—not just most of the time. Understand me?”

Robert A. Heinlein book Red Planet

“Yes, sir. I think I do.”
Source: Red Planet (1949), Chapter 2, “South Colony, Mars”, pp. 16-17

“I think that science fiction, even the corniest of it, even the most outlandish of it, no matter how badly it's written, has a distinct therapeutic value because all of it has as its primary postulate that the world does change. I cannot overemphasize the importance of that idea.”

Robert A. Heinlein

"The Discovery of the Future," Guest of Honor Speech, 3rd World Science Fiction Convention, Denver, Colorado (4 July 1941)

“It didn’t happen that way,” Roger Stone cut in, “so there is no use talking about other possibilities. They probably aren’t really possibilities at all, if only we understood it.”
Pollux: “Predestination.”
Castor: “Very shaky theory.”
Roger grinned. “I’m not a determinist and you can’t get my goat. I believe in free will.”
Pollux: “Another very shaky theory.”
“Make up your minds,” their father told them. “You can’t have it both ways.”
“Why not?” asked Hazel. “Free will is a golden thread running through the frozen matrix of fixed events.”
“Not mathematical,” objected Pollux.
Castor nodded. “Just poetry.”

Robert A. Heinlein book The Rolling Stones

“And not very good poetry.”
Source: The Rolling Stones (1952), Chapter 14, “Flat Cats Factorial” (p. 182)

“He gave up and went back to loafing, found that he could sleep all right in the afternoons but that the practice kept him awake at night.”

Robert A. Heinlein book Between Planets

Source: Between Planets (1951), Chapter 17, “To Reset the Clock” (p. 173)

“McIlvaine continued, “Take the amoeba—a more basic, and much more successful life form than than ours. The motivational psychology of the amoeba—”
I switched off my ears; free speech gives a man the right to talk about the “psychology” of an amoeba, but I don’t have to listen.”

Robert A. Heinlein book The Puppet Masters

Source: The Puppet Masters (1951), Chapter 19 (p. 104)

“Doc says the Company set-up is just one big happy family, and the idea that it is a non-profit corporation is the biggest joke since women were invented.”

Robert A. Heinlein book Red Planet

Source: Red Planet (1949), Chapter 4, “Lowell Academy”, p. 44

“Death is an old friend; I know him well. I lived with him, ate with him, slept with him; to meet him again does not frighten me—death is as necessary as birth, as happy in its own way.”

Robert A. Heinlein book I Will Fear No Evil

Source: I Will Fear No Evil (1970), Chapter 27, p. 488

“The sort of guardian you can hire is worth about as much as the sort of wife you can buy.”

Robert A. Heinlein book Space Cadet

Source: Space Cadet (1948), Chapter 12 “P.R.S. Pathfinder”, p. 143

“The United States had become a place where entertainers and professional athletes were mistaken for people of importance.”

Robert A. Heinlein book To Sail Beyond the Sunset

Source: To Sail Beyond the Sunset (1987), p. 370 (Ace 1988)

“I looked it up later; he was right. Dad is an absolute mine of useless information. He says a fact should be loved for itself alone.”

Robert A. Heinlein book Farmer in the Sky

Source: Farmer in the Sky (1950), Chapter 9, “The Moons of Jupiter” (pp. 90-91)

“The door dilated.”

Robert A. Heinlein book Beyond This Horizon

This offhand mention has become the simplest (three words!) and often-quoted exposition of the wonders of a different world, where what would be novel today has become simply the way things work.
Source: Beyond This Horizon (1948; originally serialized in 1942), Chapter 1, “All of them should have been very happy—”, p. 5 and several other times