“Reason shows us our duty; he who can make us love our duty is more powerful than reason itself.”

1960s, The American Promise (1965)

Source: Puerto Rican Catholics Urged to Open Hearts to Others at Cathedral Mass https://www.cny.org/stories/puerto-rican-catholics-urged-to-open-hearts-to-others-at-cathedral-mass,4099 (1 December 2010)

Essai sur la théorie de l'économie politique et de ses rapports avec la morale et le droit. (1867). Quoted by Teotonio R. de Souza in Indo-Portuguese history (1985), p. 210
Essai sur la théorie de l'économie politique et de ses rapports avec la morale et le droit (1867)

Vol. 2, Ch. 24 "Oracular Philosophy and the Revolt against Reason"
The Open Society and Its Enemies (1945)
Context: ... the attempt to make heaven on earth invariably produces hell. It leads to intolerance. It leads to religious wars, and to the saving of souls through the inquisition. And it is, I believe, based on a complete misunderstanding of our moral duties. It is our duty to help those who need help; but it cannot be our duty to make others happy, since this does not depend on us, and since it would only too often mean intruding on the privacy of those towards whom we have such amiable intentions.

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 197.

Source: The Ascent to Truth (1951), Ch. X : Reason in the Life of Contemplation, p. 114.
Context: One might compare the journey of the soul to mystical union, by way of pure faith, to the journey of a car on a dark highway. The only way the driver can keep to the road is by using his headlights. So in the mystical life, reason has its function. The way of faith is necessarily obscure. We drive by night. Nevertheless our reason penetrates the darkness enough to show us a little of the road ahead. It is by the light of reason that we interpret the signposts and make out the landmarks along our way.
Those who misunderstand Saint John of the Cross imagine that the way of nada is like driving by night, without any headlights whatever. This is a dangerous misunderstanding of the saint's doctrine.

The Canton, Ohio Speech, Anti-War Speech (1918)
Context: And now for all of us to do our duty! The clarion call is ringing in our ears and we cannot falter without being convicted of treason to ourselves and to our great cause.
Do not worry over the charge of treason to your masters, but be concerned about the treason that involves yourselves. Be true to yourself and you cannot be a traitor to any good cause on earth.
Yes, in good time we are going to sweep into power in this nation and throughout the world. We are going to destroy all enslaving and degrading capitalist institutions and re-create them as free and humanizing institutions. The world is daily changing before our eyes. The sun of capitalism is setting; the sun of socialism is rising. It is our duty to build the new nation and the free republic.

Ziglar has often used this saying, but it originates with Phillips Brooks, as quoted in ‪Primary Education‬ (1916) by Elizabeth Peabody.
Misattributed