I first encoutnered this quote in an article by Rev. Adam Russell Taylor, current (as of 2024) President of Sojourners. The article, titled A ‘Nonconforming Minority Can Defeat Christian Nationalism, is posted here. Here’s the full quote from the article:

One of my favorite quotes by Rev. Martin Luther King Jr. is that we will not save the world from “pending doom” through “the complacent adjustment of a conforming majority but through the creative maladjustment of a nonconforming minority.”

The quote is visible in the online collection of Dr. Kings works available through the Martin Luther King. Jr. Research and Education Institute at Stanford, in a page titled “Draft of Chapter II, ‘Transformed Nonconformist'” (and elsewhere). This sermon draft references the Biblical passage “Do not be conformed to this world, but be transformed by the renewing of your minds” from Paul’s Letter to the Romans (12:2a, NRS). With some addtional context, here it is (the strikeout and the bold text are in the original):

The most pressing need of this hour is a dedicated circle of transformed nonconformists. Today our planet teeters on the brink of atomic annihilation. Dangerous passions of pride, hatred, and selfishness still sit contentedly on the throne of our lives, and wounded truth and love are still lying prostrate on the rugged hills of nameless colonies calvaries. Men are still genuflecting before the false gods of nationalism and materialism. If our world is to be saved from its pending doom it will come not through the complacent adjustment of the conforming majority but through the creative maladjustment of a nonconforming minority. If the world is to be lifted from the morass of confusion and chaos it will be done by men who have succeeded in standing above the world so that God can lift it through them.

An adapted version of the sermon can also be read at transformingcenter.org.

Finally, the sermon is available as Chapter 2 of the book Strength to Love, a collection of sermons originally published by Dr. King in 1963, republished in 1981 as part of the King Legacy Series by Beacon Press and with a foreword by Coretta Scott King. It is available in multiple formats on Amazon and elsewhere. The Kindle version (at least) incorrectly names the Scripture reference as Matthew 10:16 rather than Romans 12:2.