“And into whatever house you shall enter, first say, ‘Peace be to this house.' And if a son of peace be there, your peace will rest on him, but if not, it shall turn to you again.”
And whenever they enter a house they are to wish peace on the house. This parallels being like lambs. And if the inhabitant proves to be a person who responds to the offer of peace (a ‘son of peace'), as revealed by his treatment of them and his response to their message, then their peace will rest on him. And if he turns out not to be so then the blessing will be recalled. It will be the same thing as the shaking of the dust off the feet. The suggestion therefore is that God will honour their call for His peace, and the true man and his family will find peace with God, while those who do not welcome them will find no peace. They have rejected the messengers of peace.
‘Peace to you' was a normal Jewish greeting. But here it becomes more than that. It becomes a spiritual weapon, and gains its significance from the status of those who say it.