“One must live as if it would be forever, and as if one might die each moment. Always both at once.”

Mark Twain in eruption: hitherto unpublished pages about men and events, 1940, Mark Twain, Bernard Augustine De Voto, Harper & brothers. This appears to be the origin of the variant:
If you would have your work last forever, and by forever I mean fifty years, it must neither overtly preach nor overtly teach, but it must covertly preach and covertly teach.
Attributed to Twain by J. Michael Straczynski in The complete book of scriptwriting, 2002, Writer's Digest Books

Source: The Other Side of Me

Source: The Tales of Alvin Maker, Seventh Son (1987), Chapter 15 (closing words).

Source: Even Cowgirls Get the Blues

Introduction (November 1970).
Deschooling Society (1971)
Context: Universal education through schooling is not feasible. It would be no more feasible if it were attempted by means of alternative institutions built on the style of present schools. Neither new attitudes of teachers toward their pupils nor the proliferation of educational hardware or software (in classroom or bedroom), nor finally the attempt to expand the pedagogue's responsibility until it engulfs his pupils' lifetimes will deliver universal education. The current search for new educational funnels must be reversed into the search for their institutional inverse: educational webs which heighten the opportunity for each one to transform each moment of his living into one of learning, sharing, and caring. We hope to contribute concepts needed by those who conduct such counterfoil research on education — and also to those who seek alternatives to other established service industries.

From Nothing Personal, a collaboration with the photographer Richard Avedon (1964). Baldwin's text for the volume can be found " here https://scholarworks.umass.edu/cgi/viewcontent.cgi?article=1042&context=cibs".

Interview in Writers at Work Third Series (1967) edited by George Plimpton
Context: One's condition on marijuana is always existential. One can feel the importance of each moment and how it is changing one. One feels one's being, one becomes aware of the enormous apparatus of nothingness — the hum of a hi-fi set, the emptiness of a pointless interruption, one becomes aware of the war between each of us, how the nothingness in each of us seeks to attack the being of others, how our being in turn is attacked by the nothingness in others.

Pt. III, st. 22
The Ballad of Reading Gaol (1898)
Source: The Ballad of Reading Gaol and Other Poems