He replied, “If you have faith as small as a mustard seed, you can say to this mulberry tree, ‘Be uprooted and planted in the sea,’ and it will obey you. (Luke 17:6, NIV)
Jesus tells the disciples they must forgive those who repent and ask for forgiveness.
They ask Jesus to give them enough faith to do such a thing.
He tells them about mustard seeds and mulberry trees.
And then he gives them a little talk about duty.
Suppose one of you has a servant plowing or looking after the sheep. Will he say to the servant when he comes in from the field, ‘Come along now and sit down to eat’? Won’t he rather say, ‘Prepare my supper, get yourself ready and wait on me while I eat and drink; after that you may eat and drink’? Will he thank the servant because he did what he was told to do? So you also, when you have done everything you were told to do, should say, ‘We are unworthy servants; we have only done our duty.’” (Luke 17:7-10, NIV)
In other words, shut up and do what you are told. Forgive.
I hear in the talk of uprooting mulberry trees exasperation and maybe even ridicule of the disciples, who ask for more faith to do something that duty alone should be enough to require of them. Stop acting like it takes some sort of moral super-heroism to forgive people. Just do it.
Jesus is remarkably unimpressed with how hard we find it to follow his commands.