Saint Jerome (347-420), priest, translator of the Bible, Doctor of the Church
Commentary on Saint Mark's Gospel, 2; PLS 2,125f.

«The unclean spirit convulsed him with a loud cry.» This was his way of expressing his distress: by convulsing him. Since he could not ruin the man's soul, the devil wrought his anger on his body. Besides, these physical manifestations were the only means he had to show that he was coming out. When the spirit of purity makes his presence known, the spirit of impurity beats a retreat...

«All were amazed and asked one another: «What is this?» Let us look at the Acts of the Apostles and the signs given by the first prophets. What did Pharaoh's magicians say when confronted by Moses' marvellous deeds? «This is the finger of God» (Ex 8,15). It was Moses who carried them out but it was another's power of they recognised. Later on the apostles performed further marvels: «In the name of Jesus Christ, rise and walk!» (Acts 3,6); «Paul... said to the spirit: 'I command you in the name of Jesus Christ to come out of this woman'» (Acts 16,18). Jesus' name is always used. Here, however, what does he himself say? «Come out of this man» without any further precision. It is in his own name that he orders the spirit to come out. «All were amazed and asked one another: 'What is this? A new teaching.'» Now, in itself, the expulsion of the demon had nothing new about it: Hebrew exorcists were doing the same thing at that time. But what does Jesus say? What is this new teaching? And where is the novelty? It is that he gives the command to the unclean spirits by his own authority, referring to no one else. He himself gives the order; he does not speak in another's name but by his own authority.