“What business do you have here with us, Jesus? I know what you're up to! You're the Holy One of God, and you've come to destroy us!"
The crowd is astounded - stunned - by the teaching of Jesus because they know him only as a peasant wood worker from Nazareth. (After all, nothing good has ever come from that part of town.)
In Mark 1:21-28, this unclean spirit is the first to "know who Jesus is," the Holy One of God, and publically acknowledge him as such.
The fact that Jesus can order the spirit to be silent and come out of the man further confirms that Jesus has spiritual authority; is a person of higher spiritual rank than the unclean spirit; is a Holy One of God.