“Even the fear of death is nothing compared to the fear of not having lived authentically and fully.”
O Magazine, May 2004
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Frances Moore Lappé 7
activist against world hunger 1944Related quotes

Not by Twain, but from Edward Abbey's A Voice Crying In The Wilderness (1989).
Misattributed

“Men fear thought as they fear nothing else on earth – more than ruin, more even than death.”
Source: 1910s, Why Men Fight https://en.wikisource.org/wiki/Why_Men_Fight (1917), pp. 178-179
Context: Men fear thought as they fear nothing else on earth – more than ruin, more even than death. Thought is subversive and revolutionary, destructive and terrible; thought is merciless to privilege, established institutions, and comfortable habits; thought is anarchic and lawless, indifferent to authority, careless of the well-tried wisdom of the ages. Thought looks into the pit of hell and is not afraid. It sees man, a feeble speck, surrounded by unfathomable depths of silence; yet it bears itself proudly, as unmoved as if it were lord of the universe. Thought is great and swift and free, the light of the world, and the chief glory of man.

“Even death is not to be feared by one who has lived wisely.”
As quoted in Wisdom for the Soul: Five Millennia of Prescriptions for Spiritual Healing (2006) edited by Larry Chang, p. 193
This is actually a pithy modern-day 'summary' of the "Abhaya Sutta" (AN 4.184). It appears in "Buddha’s Little Instruction Book" by Jack Kornfield (p88).
Unclassified

“Nothing is so much to be feared as fear. Atheism may comparatively be popular with God himself.”
September 7, 1851
Journals (1838-1859)

“People living deeply have no fear of death.”
The Diary Of Anais Nin, Volume Two (1934-1939)
Diary entries (1914 - 1974)
“Don't fear death, fear the un-lived life”
Variant: dont be afraid of death, be afraid of the unlived life.
Source: Tuck Everlasting