“One is forced to the view, for which there is so much evidence even though without rigorous scientific basis, that besides this material world another, second, purely spiritual world order exists, with just as many diversities as that in which we live-—we are to participate in it.”

17 May 1940
My Day (1935–1962)

Rediscovering Lost Values http://mlk-kpp01.stanford.edu/index.php/kingpapers/article/rediscovering_lost_values/, Sermon delivered at Detroit's Second Baptist Church (28 February 1954)
1950s

Sādhanā : The Realisation of Life http://www.spiritualbee.com/spiritual-book-by-tagore/ (1916)
Context: Though the West has accepted as its teacher him who boldly proclaimed his oneness with his Father, and who exhorted his followers to be perfect as God, it has never been reconciled to this idea of our unity with the infinite being. It condemns, as a piece of blasphemy, any implication of man's becoming God. This is certainly not the idea that Christ preached, nor perhaps the idea of the Christian mystics, but this seems to be the idea that has become popular in the Christian west.
But the highest wisdom in the East holds that it is not the function of our soul to gain God, to utilise him for any special material purpose. All that we can ever aspire to is to become more and more one with God. In the region of nature, which is the region of diversity, we grow by acquisition; in the spiritual world, which is the region of unity, we grow by losing ourselves, by uniting. Gaining a thing, as we have said, is by its nature partial, it is limited only to a particular want; but being is complete, it belongs to our wholeness, it springs not from any necessity but from our affinity with the infinite, which is the principle of perfection that we have in our soul.

Source: One-Dimensional Man (1964), p. 123

Physics in my generation (1956)
Context: America has grown by expansion in a practical vacuum; the pioneers of the West had to overcome terrific natural obstacles, but negligible human resistance. The Russia of today had to conquer not only natural but human difficulties: she had to break up the rotten system of the Czars and to assimilate backward Asiatic tribes; now she has set herself the task of bringing her brand of modernization to the ancient civilizations of the Far East. For this purpose it is indispensable to have a well-defined doctrine full of slogans, which appeals to the needs and instincts of the poverty-stricken masses. Thus one understands the power which Marx's philosophy has gained in the East.
What can we scientists do in this conflict? We can join the spiritual, religious, philosophical forces, which reject war on ethical grounds. We can even attack the ideological foundations of the conflict itself. For science is not only the basis of technology but also the material for a sound philosophy.

Reasoned Proposal to the Central Committee of the League for Peace and Freedom (1867)

Philo to Cleanthes, Part IV
Dialogues Concerning Natural Religion (1779)