We are to be lambs and that first of all tells us about our attitude. Of course this third verse is about danger and difficulty.
The simile Christ uses is designed to set us on our guard. It is an alarming picture, which takes the most innocuous and defenceless of creatures, and places them among the most savage and ravenous of creatures. Clearly the witnessing believer needs to be on his guard, and to pray earnestly for the Lord’s protection in carrying out the work. But the solution is not to retreat, because we are sent, and it is not as if the Lord who sends us does not know what we are going into. It is he that tells us of the danger we will face. He does not hesitate to go ahead and send us, despite knowing what lies ahead. The situation looks hopeless, and yet the believer will be kept, and he will survive, for the Lord is with him and this outweighs all the odds against him. The mission on which he is sent will also prove successful, and the witnessing believer will triumph over the world, though he be a lamb among wolves.
Note the contrast between verse 2 and verse 3. Verse 2 is pray, and verse 3 is go, and the application is very simple. There are some who do neither: they neither pray, nor witness themselves: that's a tragedy. Perhaps they once were better, but now they neither pray earnestly for souls – they don't have a ministry of intercession – nor do they witness themselves. Then there are some people who pray, but they don't go themselves. They do pray for souls, and that's good, but they seldom witness and they have no part in the work of the Lord. There are other rather presumptuous people who go, they witness, but they don't support their witness with prayer, and that's foolish, because it cannot be blessed. But then there are those who both pray and witness, and that's how it ought to be for us all.