They said therefore to him, "What then do you do for a sign, that we may see, and believe you? What work do you do?

 Here we come upon a truth that re-emerges in every age. Time and again it is not the intellectual difficulty which keeps men from becoming Christians; it is the height of Christ's moral demand. At the heart of an religion there must be mystery, for the simple reason that at that heart there is God. In the nature of things man cannot ever fully understand God. Any honest thinker will accept that there must be mystery. 