The words their hearts grow pliant can be glossed ‘their hearts grow less hard.’ The barrier in question may be spoken of as hardness of the heart or rust on the heart or clouds over the Moon or as a dragon that guards access to the Fountain of Life. If it were not for this barrier, there would be no need for religion in the ordinary sense, for Revelation could come directly to each man in his Heart which would then refract the Message to the mind and to the rest of the psychic substance. There would thus be a perpetual flow and ebb between the Self and the self. But as things are, a special Messenger has to be sent that he may transmit to others what his Heart receives.
– Martin Lings