"This holy Mother," he writes, "had been desirous of obtaining some insight into the beauty of a soul in grace. Just at that time she was commanded to write a treatise on prayer, about which she knew a great deal from experience. On the eve of the festival of the Most Holy Trinity she was thinking what subject she should choose for this treatise, when God, Who disposes all things in due form and order, granted this desire of hers, and gave her a subject. He showed her a most beautiful crystal globe, made in the shape of a castle, and containing seven mansions, in the seventh and innermost of which was the King of Glory, in the greatest splendour, illumining and beautifying them all. The nearer one got to the centre, the stronger was the light; outside the palace limits everything was foul, dark and infested with toads, vipers and other venomous creatures.
"While she was wondering at this beauty, which by God's grace can dwell in the human soul, the light suddenly vanished. Although the King of Glory did not leave the mansions, the crystal globe was plunged into darkness, became as black as coal and emitted an insufferable odour, and the venomous creatures outside the palace boundaries were permitted to enter the castle.
"This was a vision which the holy Mother wished that everyone might see, for it seemed to her that no mortal seeing the beauty and splendour of grace, which sin destroys and changes into such hideousness and misery, could possibly have the temerity to offend God. It was about this vision that she told me on that day, and she spoke so freely both of this and of other things that she realized herself that she had done so and on the next morning remarked to me: 'How I forgot myself last night! I cannot think how it happened. These desires and this love of mine made me lose all sense of proportion. Please God they may have done me some good!' I promised her not to repeat what she had said to anyone during her lifetime."
~Introduction to the Interior Castle, Saint Teresa of Avila
Volume 2, The Collected Works of Teresa of Avila (ICS EDITION)