What challenge?
Who says His father cannot... as in, is "physically" unable? Perhaps he cannot because the people do not want his interference, they do not want Him... and so he stays out of their lives altogether? Perhaps a sign such as that would make thing worse for a people who do not want to know Him, or be forced into knowing there is a God? Perhaps greater battles would come as people fight over the nature of God, rather than just the reality of Him? Perhaps Christ quelled the storm on faith? The same as he said we could, if we but had faith to do so. Perhaps there are a multitude of things I cannot even consider, as I keep saying. Perhaps, perhaps, perhaps... not black and white, absolute... too many variables and unknowns.
Peace,
tammy