Mark 3:13

And he goeth up into the mountain, and calleth unto him whom he himself would; and they went unto him. And he appointed twelve, that they might be with him, and that he might send them forth to preach, and to have authority to cast out demons.

The mountain ... probably refers to the elevation some five miles west of Galilee, called Mount Hatten, where it is also supposed that Jesus delivered the Sermon on the Mount. Luke adds the significant detail that Christ prayed all night before appointing the Twelve (Luke 6:13).

And they went unto him ... Bickersteth states that these words actually mean "went away to him, implying that they forsook their former pursuits."[5] This tends to remove the abruptness of the call of four apostles recounted in the first chapter and shows that Mark did not mean that at that time they forsook their occupations. This was the occasion when they gave up their fishing.

Preach ... and cast out demons ... Mark laid great stress on the mission of Christ to destroy the works of the devil. The demonic creation, under satanic domination, had doubtless learned who Christ was from the heavenly announcement: at Jesus' baptism, which must have sent a shudder of apprehension throughout the whole kingdom of evil. Satan and all of the beings in his service worked feverishly to kill Jesus, little dreaming that in the death of Christ their entire kingdom and all of its works would be overthrown.