This was the first time that Jesus had sent out His trainee apostles without His personal supervision. They were to go in twos (Mark 6:7) but without any reserve supplies, totally dependent on strangers to provide hospitality and accommodation. Their mission would be obvious - they were coming in the Name of the Lord, preaching the same gospel as Jesus and doing miracles. Would they be safe?
 
They were going into hostile territory. While there would be some people seeking God's peace, who would welcome them, others would conspire against them. It was like sheep going into a pack of wolves - their reception was potentially lethal. Jesus knew what was coming to Him, so He trained His disciples to expect the same (John 15:18-21). It was essential 'battle training' for the men who would subsequently lead the early church through many persecutions.
 
So Jesus warned them not to be naive. They must understand that they were, from a human point of view, defenceless and at risk from any aggressor. They needed shrewd wisdom ('shrewd as snakes' was a Middle Eastern proverb meaning watchful and prudent). We might say 'do not stir up a hornets' nest'. Some conflicts might be necessary (truth against falsehood) but the disciples should not be arrogant or stupidly inflame antagonism (1 Peter 4:13-16). They must be men of integrity, transparently Christ's and free from any accusation of self-serving or wickedness.
 
It is the same today. Many believers never go out with the gospel. They think that their safety lies in being 'undercover Christians': they have missed the point. The gospel is a dangerous message because it has the power to change people so that they submit to God (Romans 1:16); and many people do not want that. However if we do not go, then nobody will know the grace of Christ. Some Christians are inappropriate and undiscerning - they invite trouble unnecessarily. Others are self-serving and deserve the charge of hypocrisy. We all need to hear the words of Jesus: don't be naïve; go with a dangerous message to rebellious people; be prudent and discerning; don't invite trouble; act with integrity.