Jesus summoned the Twelve and began to send them in pairs, giving them authority over unclean spirits and to preach repentance. Having been with Jesus for some time as they listened to his teachings and watched him minister to people, the apprenticeship of the twelve came to an end. The time had come for the twelve to take their position at the side of Jesus and assume the task for which they had been “drafted.” It was not for mere companionship that Jesus had recruited the twelve. Jesus was not feeling lonely, so to speak. Jesus had recruited the twelve so that they could become co-workers with him in the vineyard of the Lord, for as Jesus would later remind the seventy-two at their own commissioning, while the harvest was plentiful, the same could not be said of the workers (cf. Luke 10:2). As Jesus went around the towns and villages proclaiming the good news of the kingdom, he got overwhelmed by the people’s thirst for the good news. The twelve were to share in the mission of Jesus. Like Jesus, the twelve were to proclaim the good news of the in-breaking of the kingdom of God. 
Whereas the mission of the twelve was multifaceted, it is worth noting that in the Gospel according to Mark, the driving out of demons and other unclean spirits is given priority (he gave them authority over unclean spirits). Casting out unclean spirits was an important part of Jesus’ mission. The kingdom of God which Jesus was proclaiming is nothing other than a world where fullness of life is present. It is a world where happiness, good health, peace, and harmony are found. In such a world, there is no room for unclean spirits and demons, for these spirits represent the absence of goodness and happiness. In the evangelist Mark’s time, unclean spirits and demons were believed to be the origin and cause of all illnesses. The primary goal of the unclean spirits was the disruption of the natural order, that is, peace and harmony in creation. Consequently, to ensure that creation enjoyed happiness, good heath, peace and harmony, the unclean spirits had to be completely destroyed. It is not a coincidence, therefore, that the first miracle that Jesus performs in the Markan account is the cure of a demoniac (cf. Mark 1:21-28). There could not have been better way for Jesus to announce the in-breaking of the kingdom of God than to free a man from a demonic possession.
There is another equally important reason why casting out evil spirits was given such a priority by Jesus. Jesus’ mission had as its goal the bringing back of creation into a life of obedience to God. The fullness of life which is the content of the good news of the kingdom of God proclaimed by Jesus and entrusted to the twelve could only be realized when creation has once again learnt to listen to the voice of God. In the Genesis account of the fall, Adam and Eve found themselves ousted from the garden of life (and subsequently from God’s life-giving presence) when they chose to listen and believe the lies of Satan, the prince of darkness and father of demonic spirits. Satan is hostile to God and is always out to frustrate God’s plans for creation. Because Satan is cunningly perceptive, he was able to have Adam and Eve believe his lies. Jesus’ mission of bringing creation back into God’s obedience cannot be successful if Satan continues to fill the hearts of men and women with lies. The obedience required for a harmonious re-unification of creation and the Creator can only exist if the unclean spirits are annihilated. The proclamation of the Good News by Jesus can only succeed and bear fruits if creation can once again listen to the voice of God without any interference.