“As a philosopher, if I were speaking to a purely philosophic audience I should say that I ought to describe myself as an Agnostic, because I do not think that there is a conclusive argument by which one prove that there is not a God. On the other hand, if I am to convey the right impression to the ordinary man in the street I think that I ought to say that I am an Atheist, because, when I say that I cannot prove that there is not a God, I ought to add equally that I cannot prove that there are not the Homeric gods.”

Source: The Hitchhiker's Guide to the Galaxy

"The Immortal" (1949)

Statement published in A Year of Beautiful Thoughts‎ (1902) by Jeanie Ashley Bates Greenough, p. 172, Third statement for June 11. This has often been misattributed to Helen Keller in some published works since at least 1980, perhaps because she somewhere quoted it.
Variant:
I am only one,
But still I am one.
I cannot do everything,
But still I can do something;
And because I cannot do everything,
I will not refuse to do the something that I can do.
The Book of Good Cheer : A Little Bundle of Cheery Thoughts‎ (1909) by Edwin Osgood Grover, p. 28; also in Masterpieces of Religious Verse (1948) by James Dalton Morrison, p. 416, where it is titled "Lend a Hand"

As quoted in a eulogy for Darrow by Emanuel Haldeman-Julius (1938)

Interview with "El País", 2009.

What is an Agnostic? (1953)
1950s

Source: Anna Karenina

Si je crois en Dieu? Oui, quand je travaille. Quand je suis soumis et modeste, je me sens tellement aidé par quelqu'un qui me fait faire des choses qui me surpassent. Pourtant je ne me sens envers lui aucune reconnaissance car c'est comme si je me trouvais devant un prestidigitateur dont je ne puis percer les tours.
1940s, Jazz (1947)