1-2 Jesus said to his disciples: ‘Things that cause people to stumble are bound to come, but woe to anyone through whom they come. It would be better for them to be thrown into the sea with a millstone tied round their neck than to cause one of these little ones to stumble.  3-4 So watch yourselves. ‘If your brother or sister sins against you, rebuke them; and if they repent, forgive them. Even if they sin against you seven times in a day and seven times come back to you saying “I repent,” you must forgive them.’

5-6 The apostles said to the Lord, ‘Increase our faith!’ 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. 7-9 ‘Suppose one of you has a servant ploughing 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? 

10 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 17:1-10 New International Version

One of life's hardest challenges is to fully forgive someone who has harmed us and damaged our relationship with God. A person may have their life strongly influenced for the worse through the actions of others, be it parents, friends, educators, or even fellow Christians. Some have been cheated in business by people who said they were Christians. Jesus touches on both sides of the problem. What’s the warning of vs 1-2?

Jesus warns that those who set out to harm Christians are in great danger. That was especially true of the unbelieving Jews and Romans who persecuted the Church while it was still young. But it's also a reminder that we must take great care that we are not the cause for the faith of new or weaker Christians to waver.

But what’s the other side of this problem? How are Christians to respond to those who had really hurt them through words or actions, but who apologize and appear to be truly sorry. (vs 3-4)

Clearly we are called to forgive those people. And notice that it wasn't just a suggestion by Jesus, it was an order! He says 'you must forgive them'. We may know from personal experience how difficult that can be. If the thing done to us caused deep hurt and perhaps affected the course of our life, it's not easy to simply forgive. Perhaps this what led to the disciples asking Jesus to 'increase their faith'. It may've been a general request on their part, but the fact that Luke brings it in here suggests to me that they were realizing how difficult following Jesus could turn out to be.

Jesus replies with a short comment about faith, and then a longer comment about servants and their master. What do you think this would have meant to the disciples?

My thoughts are that He was saying that the real issue here was not faith, but obedience. It's not that they didn't have enough faith to be able to forgive those who hurt them and caused them to stumble; they just needed to remember that this was an order they needed to obey. Jesus said that even a little faith can accomplish more than we can imagine or expect. The more difficult thing in life is to obediently live out the words that Jesus taught. That includes the difficult task of forgiving people who really hurt us when they ask for our forgiveness.

Verse 10 is quite humbling. It shows us that Jesus knows that at the end of the day most of us will have fallen well short of the sort of obedience to His words we should have given. And in those cases where we did do the right thing, we've nothing to boast about, we were simply following orders and doing our duty. Yes, we will meet up with people who are going to be a test to our faith. But we have a loving saviour who is with us in such situations. It's as if He says to us “Trust me, and do what I've told you”. That may be hard at times, but it's the right way to go.