two by two: This practice fulfilled the Old Testament rule of needing two witnesses (Numbers 35:30; Deuteronomy 17:6; 19:15; see Matthew 18:16). It also allowed for them to have companionship and mutual help Acts 13:1–3; 15:22, 39, 40).
giving them authority: Jesus can give his authority to others (see Mark 1:27; 2:10).
Jesus told the disciples to drive out unclean spirits and to preach and heal (6:12–13; see 6:30).