Because they knew him

"When it was evening, after sunset, they brought to him all who were ill or possessed by demons. The whole town was gathered at the door. He cured many who were sick with various diseases, and he drove out many demons, not permitting them to speak because they knew him. " - Mark 1:32-34

What strikes me about these verses is not that the demons Jesus drove out couldn't speak, but that Mark takes time to note the reason - "because they knew him". I sometimes think of Jesus as just a man - a very special man - but just a man. But Mark clearly notes that they knew him - demons (in the first century sense) don't react that way to just anyone. If they did, they would have been cured by other men. But they recognize Jesus for who he really is - the Messiah, God made flesh, the real deal. And they don't dare challenge Jesus, but rather are forced to obey.

Perhaps it's because of the showdown between Satan and Jesus in the desert. While Mark doesn't go into detail about it, Luke certainly does, describing three of the many temptations Jesus is faced, and how he uses scripture to best Satan at every argument. If the dark one cannot affect Jesus, certainly these demons have no chance!

And that leads me to the most striking revelation about these three verses - even in demons we can find witness to the awesome power of Christ!