“And, behold, they cried out, saying, What have we to do with thee, Jesus, thou Son of God? art thou come hither to torment us before the time?”
And behold, they cried out, What have you to do with us, O Son of God? Have you come here to torment us before the time? The demons recognize Jesus immediately — Son of God, the title the voice at the baptism used — and ask the right theological question: have you come before the time? The demons know there is an appointed time for their judgment and destruction (Revelation 20:10); they are asking whether Jesus is initiating the final judgment ahead of schedule. The torment they fear is not the exorcism but the eschatological destruction. Their fear communicates their own understanding of Jesus' identity: they know who he is and what his presence means for their future.
Church Fathers on Matthew 8:29
Because there were who thought Christ to be a man, therefore the daemons came to proclaim His divinity, that they who had not seen the sea raging and again still, might hear the daemons crying; 'And when he was come to the other side in the country of the Gergesenes, there met him two men having daemons.'
Gerasa is a town of Arabia beyond Jordan, close to Mount Gilead, which was in the possession of the tribe of Manasseh, not far from the lake of Tiberias, into which the swine were precipitated.
Whereas Matthew relates that there were two who were afflicted with daemons, but Mark and Luke mention only one, you must understand that one of them was a person of note, for whom all that country was in grief, and about whose recovery there was much care, whence the fame of this miracle was the more noised abroad.