“As is our confidence, so is our capacity.”

Also translated as: It is not because things are difficult that we do not dare, but because we do not dare, things are difficult.
Letter CIV, verse 26
Epistulae Morales ad Lucilium (Moral Letters to Lucilius)
Context: But how much more highly do I think of these men! They can do these things, but decline to do them. To whom that ever tried have these tasks proved false? To what man did they not seem easier in the doing? Our lack of confidence is not the result of difficulty. The difficulty comes from our lack of confidence.

The 8th Habit : From Effectiveness to Greatness‎ (2004), p. 63
The 8th Habit : From Effectiveness to Greatness‎ (2004)

1900s
Context: When Mr. Cobden preached his doctrine he believed, as he had at that time considerable reason to suppose, that while foreign countries would supply us with our foods and raw materials we should remain the workshop of the world and should send them in exchange our manufactures. But that is exactly what we have not done. On the contrary... we are sending less and less of our manufactures to them, and they are sending more and more of their manufactures to us... Our existence as a nation depends upon our manufacturing capacity and production.

Speech in Glasgow (6 October 1903), quoted in The Times (7 October 1903), p. 4.

1940s, Response to the attack on Pearl Harbor (1941)

Manuscript Found in Accra (2012), What should survivors tell their children?

Source: Man’s Search for Himself (1953), p. 138

Statement (1853) as quoted in Andrew Johnson, Plebeian and Patriot (1928) by Robert Watson Winston.
Quote
Context: There are some who lack confidence in the integrity and capacity of the people to govern themselves. To all who entertain such fears I will most respectfully say that I entertain none... If a man is not capable, and is not to be trusted with the government of himself, is he to be trusted with the government of others... Who, then, will govern? The answer must be, Man — for we have no angels in the shape of men, as yet, who are willing to take charge of our political affairs.

Source: Travels with Charley: In Search of America