Green and living trees can absorb the bounty of the sun; dead roots and withered branches are destroyed by it. Therefore, man must seek capacity and develop readiness. As long as he lacks susceptibility to divine influences, he is incapable of reflecting the light and assimilating its benefits. Sterile soil will produce nothing, even if the cloud of mercy pours rain upon it a thousand years. We must make the soil of our hearts receptive and fertile by tilling in order that the rain of divine mercy may refresh them and bring forth roses and hyacinths of heavenly planting.
‘Abdu’l-Baha, The Promulgation of Universal Peace. (Wilmette: US Baha’i Publishing Trust, 1982 second edition) P. 148. Available from http://reference.bahai.org/en/t/ab/PUP/pup-55.html#pg148