“Behold, I send you forth as sheep in the midst of wolves, be you therefore wise as serpents, and harmless as doves.”
Jesus introduces His warnings about the future by likening their going forth as similar to that of sheep going among wolves. This in itself is a reminder of the uniqueness of the event. No one sends sheep among wolves. Only God could do that. As those who are poor in spirit (Matthew 5:3) and meek (Matthew 5:5) and compassionate (Matthew 5:44) they will find themselves having to face the arrogant, the proud and even the violent. They will thus need to be shrewd, to know how to pacify, and to avoid confrontation. The vivid contrast demonstrates that He is speaking in exaggerated fashion. The extremes would not always be quite as great. This was the worst case scenario (but they would often feel like sheep among wolves).
‘Wise as serpents.' The serpent keeps out of sight and moves around unobserved, and then suddenly it strikes. Both its safety and its effectiveness depend on its subtlety, its seeming non-existence, and its speed of action. In Genesis 49:17-18 it is thus linked interestingly with salvation. Its subtlety is proverbial.
‘Harmless/innocent as doves.' The dove is harmless, causes no trouble, is sweet and gentle (and thus used of Solomon's beloved - Song of Solomon 1:15; Song of Solomon 2:14; Song of Solomon 4:3 etc), open and honest, and is non-violent. The combination of subtlety and wisdom, swiftness and sweetness, rapid reaction and gentleness, reveals those who are shrewd, wise, and quick and decisive in thought and action, and yet reliable, gentle, honest, kind and compassionate.