“Education is the key; only when you've got the MASTER key to unlock all the locks!”

Quantum Mechanics, The Key to Understanding Magnetism, Nobel Lecture http://nobelprize.org/nobel_prizes/physics/laureates/1977/vleck-lecture.pdf (December 8, 1977)

Ben Yamen's Song of Geometry (1853)
Context: The Key! it is of wonderful construction, with its infinity of combination, and its unlimited capacity to fit every lock. … it is the great master-key which unlocks every door of knowledge and without which no discovery which deserves the name — which is law, and not isolated fact — has been or ever can be made. Fascinated by its symmetry the geometer may at times have been too exclusively engrossed with his science, forgetful of its applications; he may have exalted it into his idol and worshipped it; he may have degraded it into his toy... when he should have been hard at work with it, using it for the benefit of mankind and the glory of his Creator.

Canto XIII, lines 58–60 (tr. C. E. Norton).
The Divine Comedy (c. 1308–1321), Inferno

Source: Little Women

Song lyrics, Biograph (1985), Up to Me (recorded 1974)

Entry (1962)
Eric Hoffer and the Art of the Notebook (2005)

First mentioned as "Continuous effort — not strength or intelligence — is the key to unlocking and using our potential." according to Quote Investigator in the 1981 book The Reflecting Pond: Meditations for Self-Discovery by Liane Cordes, Quote Page 89, Hazelden Publishing, Center City, Minnesota. For further research on this quote see: Quote Investigator (August 31, 2013): Continuous Effort — Not Strength or Intelligence — Is the Key to Unlocking and Using Our Potential Winston Churchill? Liane Cordes? Liane Cardes? Apocryphal? Archived http://archive.is/E0M12 on June 2, 2020.
Source: https://quoteinvestigator.com/2013/08/21/effort/ from the original

Essays on Men and Manners (1804)