“There is an innocence in admiration: it occurs in one who has not yet realized that they might one day be admired.”

Other Inquisitions (1952), The Modesty of History
Context: Only one thing is more admirable than the admirable reply of the Saxon king: that an Icelander, a man of the lineage of the vanquished, has perpetuated the reply. It is as if a Carthaginian had bequeathed to us the memory of the exploit of Regulus. Saxo Grammaticus wrote with justification in his Gesta Danorum: "The men of Thule [Iceland] are very fond of learning and of recording the history of all peoples and they are equally pleased to reveal the excellences of others or of themselves."
Not the day when the Saxon said the words, but the day when an enemy perpetuated them, was the historic date. A date that is a prophecy of something still in the future: the day when races and nations will be cast into oblivion, and the solidarity of all mankind will be established.

As quoted in The Modern Handbook of Humor (1967) by Ralph Louis Woods
Variants:
A conservative is someone who admires radicals a century after they're dead.
A conservative is one who admires radicals centuries after they're dead.

St. 3
(1819)

The Education of Henry Adams (1907)

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

Farewell Address (2003)

Ariel (1923)