“The faith commended to their faithful by all religions, and considered by them as a virtue essential for him who hopes for eternal salvation, is nowise approved in the Secret Teachings. Based on the advice given by the Buddha to His disciples, the primary recommendation that the Masters give to neophytes is: Doubt! Doubt is an incitement to research, and research is the Way which leads to Knowledge.”
Source: The Secret Oral Teachings in the Tibetan Buddhist Sects (1964)
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Alexandra David-Néel 5
French explorer, spiritualist, Buddhist, anarchist and writ… 1868–1969Related quotes

“Faith which does not doubt is dead faith.”
Fe que no duda es fe muerta.
La Agonía del Cristianismo (The Agony of Christianity) (1931)
Source: Jesus Before Christianity: The Gospel of Liberation (1976), p. 42.
Context: There can be no doubt that Jesus was a remarkably cheerful person and that his joy, like his faith and hope, was infectious. This was in fact the most characteristic and most noticeable difference between Jesus and John. As we shall see later, Jesus feasted while John fasted (Luke 7:31-34).
Source: The Christian Agnostic (1965), p.227 [ellipsis added]

Lecture on Bhagavad-gita, Chapter 7, verse 18; New York; http://prabhupadabooks.com/classes/bg/7/18/new_york/october/12/1966?d=1 (12 October 1966)

Der Glaube an eine bestimmte Idee gibt dem Forscher den Rückhalt für seine Arbeit. Ohne diesen Glauben wäre er verloren in einem Meer von Zweifeln und halbgültigen Beweisen.
Attributed in Konrad Zuse http://www.dpma.de/ponline/erfindergalerie/bio_zuse.html on "Die Erfindergalerie", dpma.de, 2008

Source: Guide for the Perplexed (c. 1190), Part III, Ch.20
Context: Consider in how many ways His knowledge is distinguished from ours according to all the teaching of every revealed religion. First, His knowledge is one, and yet embraces many different kinds of objects. Secondly, it is applied to things not in existence. Thirdly, it comprehends the infinite. Fourthly, it remains unchanged, though it comprises the knowledge of changeable things; whilst it seems that the knowledge of a thing that is to come into existence is different from the knowledge of the thing when it has come into existence; because there is the additional knowledge of its transition from a state of potentiality into that of reality. Fifthly, according to the teaching of our Law, God's knowledge of one of two eventualities does not determine it, however certain that knowledge may be concerning the future occurrence of the one eventuality.

Major Richard Sharpe (describing his murdered wife, Teresa Moreno) p. 339
Sharpe (Novel Series), Sharpe's Enemy (1984)