From his 1922 book, What I Saw in America (Hodder & Stoughton),
The devil can quote Scripture for his purpose; and the text of the Scripture which he now most commonly quotes is, “The kingdom of heaven is within you.” That text has been the stay and support of more Pharisees and prigs and self-righteous spiritual bullies than all the dogmas in creation; it has served to identify self-satisfaction with the peace that passes all understanding. And the text to be quoted in answer to it is that which that no man can receive the kingdom except as a little child. What we are to have inside is the childlike spirit; but the childlike spirit is not entirely concerned about what is inside. It is the first mark of possessing it that one is interested in what is outside. The most childlike thing about a child is his curiosity and his appetite and his power of wonder at the world. We might almost say that the whole advantage of having the kingdom within is that we look for it somewhere else.
Read whole thing here.