St. John of the Cross compares man to a window through which the light of God is shining. If the windowpane is clean of every stain, it is completely transparent, we do not see it at all: it is “empty” and nothing is seen but the light. But if a man bears in himself the stains of spiritual egotism and preoccupation with his illusory and exterior self, even in “good things,” then the windowpane itself is clearly seen by reason of the stains that are on it. Hence if a man can be rid of the stains and dust produced within him by his fixation upon what is good and bad in reference to himself, he will be transformed in God and will be “one with God." 

~ Thomas Merton from Zen and the Birds of Appetite.