What does this quote mean?

“The devil can cite Scripture for his purpose.”

Even respected language can be quoted selectively, so a citation should be judged by context and purpose rather than authority alone.

Source note: The Merchant of Venice, Act 1, Scene 3.

“Nothing is so firmly believed, as what we least know.”

Certainty can be strongest where understanding is weakest, so confidence by itself is poor evidence that a belief has been examined well.

Source note: Essays of Montaigne, Book I, Charles Cotton translation, in a skeptical discussion of confident claims.

“Convictions are more dangerous enemies of truth than lies.”

Nietzsche warns that fixed certainty can block revision more effectively than an obvious falsehood because conviction can make contrary evidence harder to examine.

Source note: Human, All-Too-Human, Part I, aphorism 483, “The Enemies of Truth”; Helen Zimmern translation, 1909 edition.

“There is no pre-established harmony between the promotion of truth and the welfare of mankind.”

Nietzsche rejects the comforting assumption that discovering truth must always coincide with human comfort or advantage.

Source note: Human, All-Too-Human, Part I, aphorism 517, “A Fundamental Insight”; Helen Zimmern translation.

“To define is to limit.”

Definitions clarify ideas but can also narrow them, especially when a label is treated as the complete reality of a person or experience.

Source note: The Picture of Dorian Gray, 1891 edition, Chapter 17; Lord Henry answers the Duchess of Monmouth’s question “What are you?” with “To define is to limit.”

“Truth happens to an idea.”

The compact formulation summarizes James’s pragmatic account in which an idea’s truth is connected with processes of verification and validation rather than treated only as a static label.

Source note: Pragmatism, Lecture VI, “Pragmatism’s Conception of Truth”; Project Gutenberg #5116. Capitalization has been normalized from “Truth HAPPENS to an idea”; the surrounding passage explains verification and validation.