Just because you have the authority to do something doesn’t mean that you have the strength in and of yourself to complete it. The gym may give you authority to lift as much weight as you can, but that doesn’t mean you have the strength to actually lift it. A store gives you the authority to purchase as much as you like, but you might not have the financial power to do so.
Rodney Cooper points out that the difference between authority and power as seen in Mark chapter 9.
“Jesus had given the disciples authority to cast our demons. Why, then, could they not cast this one out… They were relying on the fact that Jesus had given them authority. But he had not given it to them apart from God.”Rodney Cooper, Holman New Testament Commentary: Mark, p. 150
Maybe this was a more powerful demon… maybe they thought they could do it on their own or hadn’t thought to seek Jesus as the source of power… maybe they just needed a reminder that, as Rodney Cooper points out from John 15:5,
“Apart from me you can do nothing.”John 15:5
Jesus calls us to do things — He has work for us to do — but He also expects us to do those things in His strength. What does this mean for us? The bigger the vision, the greater the need for prayer and the greater need for reliance on Him to do in us what we cannot do in and of ourselves.