“The acquisition of any knowledge is always of use to the intellect, because it may thus drive out useless things and retain the good. For nothing can be loved or hated unless it is first known.”

'Where Do We Go From Here?" as published in Where Do We Go from Here : Chaos or Community? (1967), p. 62; many statements in this book, or slight variants of them, were also part of his address Where Do We Go From Here?" which has a section below. A common variant appearing at least as early as 1968 has "Returning violence for violence multiplies violence..." An early version of the speech as published in A Martin Luther King Treasury (1964), p. 173, has : "Returning hate for hate multiplies hate..."
1960s
Source: A Testament of Hope: The Essential Writings and Speeches
Context: The ultimate weakness of violence is that it is a descending spiral, begetting the very thing it seeks to destroy. Instead of diminishing evil, it multiplies it. Through violence you may murder the liar, but you cannot murder the lie, nor establish the truth. Through violence you may murder the hater, but you do not murder hate. In fact, violence merely increases hate. So it goes. … Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that.

Sādhanā : The Realisation of Life http://www.spiritualbee.com/spiritual-book-by-tagore/ (1916)

Thought and Word, viii
The Note-Books of Samuel Butler (1912), Part VII - On the Making of Music, Pictures, and Books

Aleph (2011)

Source: Black Blood

Source: The Complete Poems

Life and Doctrine

This passage contains some phrases King later used in "Where Do We Go From Here?" (1967) which has a section below.
1950s, Loving Your Enemies (Christmas 1957)
Variant: Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate, only love can do that.
Source: Mentioned in "Out of Osama's Death, a Fake Quotation Is Born" by Megan McArdle, The Atlantic (May 2011) http://www.theatlantic.com/national/archive/2011/05/out-of-osamas-death-a-fake-quotation-is-born/238220/, and widely distributed on twitter http://twitter.com/#!/jmadly/status/65314784136011776 as a quote of King, after the death of Osama bin Laden, the first sentence is one written by Jessica Dovey http://i.imgur.com/cqtjw.jpg on her Facebook page, which became improperly combined by others with genuine statements of King, whom she quoted, and which occur in Strength to Love (1963), Ch. 5 : Loving your enemies, and in Where Do We Go from Here : Chaos or Community? (1967), p. 62.
For the full story see "Anatomy of a Fake Quotation" by Megan McArdle, The Atlantic (May 3, 2011) http://www.theatlantic.com/national/archive/2011/05/anatomy-of-a-fake-quotation/238257/ and for the Facebook version of the quote see Did Martin Luther King, Jr. say that “I will not rejoice in the death of one, not even an enemy”? at skeptics.stackexchange.com http://skeptics.stackexchange.com.
Context: Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction.
Context: Let us move now from the practical how to the theoretical why: Why should we love our enemies? The first reason is fairly obvious. Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction. So when Jesus says "love your enemies," he is setting forth a profound and ultimately inescapable admonition. Have we not come to such an impasse in the modern world that we must love our enemies-or else? The chain reaction of evil-Hate begetting hate, wars producing more wars-must be broken, or we shall be plunged into the dark abyss of annihilation.
Context: I mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy. Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that.

Speech to the assembled White House staff before his final departure (9 August 1974)
1970s