Mark 6:7
Calling the Twelve to him, he began to send them out two by two and gave them authority over impure spirits — the mission of the twelve is the practical extension of the appointment of chapter 3. Two by two provides mutual support, accountability, and the two-witness requirement of Deuteronomy 19:15 for the validity of testimony. The authority given is specifically over impure spirits — the same authority exercised by Jesus throughout chapters 1–5. The disciples are not sent to do something new but to extend what they have been watching Jesus do. The authority is delegated and derived: they can cast out demons because Jesus gives them power to do so.