“Of all the frictional resistances, the one that most retards human movement is ignorance, what Buddha called 'the greatest evil in the world.' The friction which results from ignorance… can be reduced only by the spread of knowledge and the unification of the heterogeneous elements of humanity. No effort could be better spent.”

"The Death of Me", p. 150
Awareness (1992)
Context: Can one be fully human without experiencing tragedy? The only tragedy there is in the world is ignorance; all evil comes from that. The only tragedy there is in the world is unwakefulness and unawareness. From them comes fear, and from fear comes comes everything else, but death is not a tragedy at all. Dying is wonderful; it's only horrible to people who have never understood life. It's only when you're afraid of life that you fear death. It's only dead people who fear death.

Source: In Praise of Philosophy (1963), p. 5
Context: Even those who have desired to work out a completely positive philosophy have been philosophers only to the extent that, at the same time, they have refused the right to install themselves in absolute knowledge. They taught not this knowledge, but its becoming in us, not the absolute but, at most, our absolute relation to it, as Kierkegaard said. What makes a philosopher is the movement which leads back without ceasing from knowledge to ignorance, from ignorance to knowledge, and a kind of rest in this movement.

Mother India's Lighthouse: India's Spiritual Leaders (1971)

Socrates II: xxxi http://www.perseus.tufts.edu/hopper/text?doc=D.+L.+2.5.31&fromdoc=Perseus%3Atext%3A1999.01.0257#note-link14. Original Greek: ἓν μόνον ἀγαθὸν εἶναι, τὴν ἐπιστήμην, καὶ ἓν μόνον κακόν, τὴν ἀμαθίαν
Diogenes Laertius
Variant: The only good is knowledge and the only evil is ignorance.

The Glass Bead Game (1943)
Context: To be capable of everything and do justice to everything, one certainly does not need less spiritual force and èlan and warmth, but more. What you call passion is not spiritual force, but friction between the soul and the outside world. Where passion dominates, that does not signify the presence of greater desire and ambition, but rather the misdirection of these qualities toward an isolated and false goal, with a consequent tension and sultriness in the atmosphere. Those who direct the maximum force of their desires toward the center, toward true being, toward perfection, seem quieter than the passionate souls because the flame of their fervor cannot always be seen.

1 Corinthians 8:1
Source: Holy Hesychia: The Stillness that Knows God, p. 33

The words of Socrates, as quoted by Diogenes Laertius.
Misattributed

Socrates, 14.
The Lives and Opinions of Eminent Philosophers (c. 200 A.D.), Book 2: Socrates, his predecessors and followers