Jesus rebuked the demon, and it came out of the boy, and he was healed at that moment. He replied, “Because you have so little faith. Truly I tell you, if you have faith as small as a mustard seed, you can say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.”
Matthew 17:18, NIV
So, Jesus says that it takes faith to cast out demons, not God’s will, but man’s will.
Next Jesus says with even more faith you can tell a mountain to cast itself into the sea.
In laymen terms.
Casting out demons is level 1 for faith. After this, move on to greater heights of power. Casting out demons is spiritual cannon fodder. After you can mow down the fodder, move on to more different boss targets.
In other words, casting out demons is not the height of spiritual power, but near the beginning. Why? Because Jesus has already defeated them and made demons a spectacle of weakness and shame. When we cast out demons, we are defeating a wounded foe.