“Necessity is the most powerful divinity the world knows--it is the result of physical forces set in operation by ethical forces.”

Source: Anarcho-Syndicalism (1938), Ch. 1 "Anarchism: Its Aims and Purposes"
Context: Power operates only destructively, bent always on forcing every manifestation of life into the straitjacket of its laws. Its intellectual form of expression is dead dogma, its physical form brute force. And this unintelligence of its objectives sets its stamp on its supporters also and renders them stupid and brutal, even when they were originally endowed with the best of talents. One who is constantly striving to force everything into a mechanical order at last becomes a machine himself and loses all human feeling.
It was from the understanding of this that modern Anarchism was born and now draws its moral force. Only freedom can inspire men to great things and bring about social and political transformations. The art of ruling men has never been the art of educating men and inspiring them to a new shaping of their lives. Dreary compulsion has at its command only lifeless drill, which smothers any vital initiative at its birth and can bring forth only subjects, not free men. Freedom is the very essence of life, the impelling force in all intellectual and social development, the creator of every new outlook for the future of mankind. The liberation of man from economic exploitation and from intellectual and political oppression, which finds its finest expression in the world-philosophy of Anarchism, is the first prerequisite for the evolution of a higher social culture and a new humanity.

Variant: that love is the most powerful force in the world. That love can do anything.
Source: City of Fallen Angels

Source: Lover Revealed

Variants: "... is man’s greatest invention" and "... is the eighth wonder of the world".
May add: "He who understands it, earns it; he who doesn’t, pays it."
This Snopes article http://www.snopes.com/quotes/einstein/interest.asp concluded that its status was uncertain, while this post from The Quote Investigator http://quoteinvestigator.com/2011/10/31/compound-interest/ concludes it is most likely a false attribution, since variants of the quote date back to at least 1916, with the early variants not being attributed to Einstein.
Disputed

1990s, A Distinctly American Internationalism (November 1999)

Source: Isis Unveiled, Volume I, Chapter VI

[George Sarton, A guide to the history of science: a first guide for the study of the history of science, with introductory essays on science and tradition, Chronica Botanica Co., 1952, 3]