Luke presents two missions undertaken by Jesus’s disciples under instructions from the Master. One mission is of the Twelve, and the other of the Seventy (or Seventy-two).
Why Seventy-two? This was a symbolic number in Israeli history: in the book of Genesis, there’s a list of 72 nations in the world; there are 72 elders who assisted Moses in administering the Israeli people, 6 from each of the 12 tribes; and the first authoritative translation of the Hebrew scriptures into popular Greek was done by 72 scholars. Whence its name, the Septuagint version.
More pertinently, all that Jesus said to the Twelve is repeated and extended in scope by the early Church in commissioning the first mission bands of apostles. What Jesus demanded of his Twelve apostles now applies to missionaries everywhere.
To show their complete reliance on God, they must take just the bare minimum for themselves on the journey: neither staff nor satchel nor shoes on their feet, neither bread or money, nor even a cloak against the weather. Their trust in God was to be total.
On arrival in a village, they were to select a place to stay, and stay there only, not moving from house to house. They are to eat what they are served. Jewish dietary laws have no force any more.
The apostles should enter the home of each believer and say, “Peace upon this house,” and their blessings would descend upon the family. Their preaching affirms the presence of God’s reign among us – that is, relationships of justice, peace, fellowship, self-restraint, generosity. A presence confirmed by the miracles of healing they would work.
Finally, the injunction about shaking off the dust from their feet as a warning to those who do not receive them. The messenger of the Good News not only brings a blessing to those who welcome him, but also a judgment upon those who reject his word.