Mark 3:15

To have power [ε χ ε ι ν ε ξ ο υ σ ι α ν]. Note that he does not say to preach and to cast out, but to preach and to have authority to cast out. The power of preaching and the power of exorcising were so different that special mention is made of the divine authority with which they would need to be clothed. The power of driving out demons was given that they might apply it in confirmation of their teaching. Compare 16 20.