“Of all things love is the most potent.”

Un Art de Vivre (The Art of Living) (1939), The Art of Loving

God Knows (1984)

Source: Rules for Radicals: A Practical Primer for Realistic Radicals (1971), p. 128

1950s, Loving Your Enemies (Christmas 1957)
Context: Love is the most durable power in the world. This creative force, so beautifully exemplified in the life of our Christ, is the most potent instrument available in mankind's quest for peace and security. Napoleon Bonaparte, the great military genius, looking back over his years of conquest, is reported to have said: "Alexander, Caesar, Charlemagne and I have built great empires. But upon what did they depend? They depended on force. But centuries ago Jesus started an empire that was built on love, and even to this day millions will die for him." Who can doubt the veracity of these words. The great military leaders of the past have gone, their empires have crumbled and burned to ashes. But the empire of Jesus, built solidly and majestically on the foundation of love, is still growing. It started with a small group of dedicated men, who, through the inspiration of their Lord, were able to shake the hinges form the gates of the Roman Empire, and carry the gospel into all the world. Today the vast earthly kingdom of Christ numbers more than 900,000,000 and covers every land and tribe.

Source: De architectura (The Ten Books On Architecture) (~ 15BC), Book II, Chapter VIII, Sec. 10
Context: At Halicarnassus, the house of that most potent king Mausolus, though decorated throughout with Proconnesian marble, has walls built of brick which are to this day of extraordinary strength, and are covered with stucco so highly polished that they seem to be as glistening as glass. That king did not use brick from poverty; for he was choke-full of revenues, being ruler of all Caria.

White Racism and Black Consciousness
I Write What I Like (1978)
Variant: The greatest weapon in the hand of the oppressor is the mind of the oppressed.

As quoted in Notes for a Memoir : On Isaac Asimov, Life, and Writing (2006) by Janet Jeppson Asimov, p. 58
General sources
Variant: Properly read, the Bible is the most potent force for atheism ever conceived.
Context: If you suspect that my interest in the Bible is going to inspire me with sudden enthusiasm for Judaism and make me a convert of mountain‐moving fervor and that I shall suddenly grow long earlocks and learn Hebrew and go about denouncing the heathen — you little know the effect of the Bible on me. Properly read, it is the most potent force for atheism ever conceived.

On Fairy-Stories (1939)