The mystery of doubt

There is a line in today's Gospel that has always unnerved me a little. Mark is giving a kind of recap of the appearances of the Risen Christ: first to Mary Magdalen, then to the two disciples on the road, and finally to the Eleven (significantly enough, "while they were at table"). And the first thing he does is scold them for not accepting the testimony of the earlier witnesses! "He reproved them for their hardness of heart, not believing those who had seen him." And no sooner does he reproach them, then he entrusts them with a mission to the whole world!
It is very close to Matthew's telling, in which Jesus appears "on the mountain" (that unnamed "mountain of the Lord" that has a role in so many passages of Matthew), and when they saw him "they worshiped, but they doubted." And Jesus told those worshipful doubters, "Go into the whole world and make disciples of all nations..."
It would almost seem as if their doubt, lack of faith and hard-heartedness were almost necessary if they were going to be effective in presenting the message! We can see an extreme example of this in St. Paul, who pointed to his own unbelief as evidence of God's abounding grace.
As I've heard it said in other contexts, "God doesn't choose the qualified; he qualifies the chosen!"