"The heart of this people is waxed gross, and their ears are dull of hearing, and their eyes have they closed; lest they should see with their eyes, and hear with their ears, and understand with their heart, and should be converted, and I should heal them." (Bible)
Has anything changed in the thousands of years since this was first stated? What is it that is required for receptivity? Considering the tenacity of the dullness in the human condition, don't expect a worldly reasonable answer!
Audio of explanatory selections from S&H