Demon
The contemporary Roman Catholic Church unequivocally teaches that angels and demons are real beings rather than just symbolic devices. The Catholic Church has a cadre of officially sanctioned exorcists which perform many exorcisms each year. The exorcists of the Catholic Church teach that demons attack humans continually but that afflicted persons can be effectively healed and protected either by the formal rite of exorcism, authorized to be performed only by bishops and those they designate, or by prayers of deliverance, which any Christian can offer for themselves or others.
Mark 1
He then performs an exorcism on a demon possessed man. Curing people, especially possessed people, will be another major method Jesus uses in his mission in Mark. The demon recognizes Jesus as "the Holy one of God", the first time Jesus' supernatural opponents are shown to know his true identity. Jesus simply says "Be quiet, and come out of him!", healing the man with words alone. The people are amazed. This follows a path Mark often uses, that of a description of the affliction, Jesus' cure of the affliction, and then a demonstration of the cure to others. The power of Jesus' word over the demon might be Mark's way of trying to show to his audience, perhaps under the threat of persecution, that Jesus' message will overcome evil. By showing Jesus' teaching first before his exorcism Mark might be placing emphasis on Jesus' teachings as more important that any miracle he could perform.