Though the words of verses 13-15 are very similar to Matthew 11:20-23, verse 16 varies a little from Christ’s words (probably spoken on a separate occasion) in Matthew 11:24. The negative side of the statement is added in Luke’s record.
The principle here is that we and our work are really significant. ‘Oh I am a witness. I will witness to this person; I will witness to that person. It would be good if they listened; it would be good if they were converted.’ No, it is more important than that God is dealing with that person through you. On the basis of their acceptance or rejection of your message, they will either go to heaven or to hell. What they do with you, they do with Christ, and if they turn against you and persecute you, this is a mightily and eternally significant thing. You are seeing played out before your eyes the sealing of their eternal destiny, unless of course it is in the will of God to give them another opportunity with another messenger. But we realize our work is hugely significant, so we pray all the more and we are very diligent in how we do it.