It was the 'inner' twelve who came to Jesus, aware that their faith was weak.  It seems that they, at least partly, were beginning to understand just what it meant to be a follower of Jesus. They had seen him perform many miracles and were in awe of the faith he displayed.  Maybe they wanted to be ads much like him as they could - and having faith like his was a sgttep in that direction.  But they didn't know how to develop that kind of faith.So, they came to the 'expert' - where we all should go - for help.

Quest Study Bible note, p 1449.  The disciples thought their faith was inadequate probably because they saw how impossible it would be to do on their own what Jesus told them about forgiveness (vv 3-4) ....  The disciples most likely recognized that there was a link between their substandard actions and a shortfall in their faith.

V 6.
and the Lord replied, 'If you had faith no bigger even than a mustard-seed, you could say to this mulberry tree, "Be uprooted and replanted in the sea", and it would at once obey you.

This statement reminds me of a similar one reported in Matthew 17: 20 where Jesus says to his disciples who had been trying to exorcise a demon from a boy and failed: 'I tell you this: if you have faith no bigger than a mustard-seed, you will say to this mountain, "move from here to there!", and it will move;  nothing will prove impossible to you.' When I was a child, this statement impressed me greatly. It made me aware of how much I needed to trust God, as I just couldn't imagine a mountain up and moving, just because I had told it to!  I knew God could do that if he wanted to, but I couldn't see it happening if I said so. Ever since then I have prayed for my faith to grow stronger - and I believe God is gradually answering that prayer. (Not that I particularly want to move physical mountains or mulberry trees!)  Nothing I can do will increase my faith - that is God's work, by showing me over and over again how much he is to be trusted.

Quest Study Bible note on Matthew 17: 20.p 1359.  Faith was a struggle for (the disciples), just as it is for us today.  Yet lack of faith creates obstacles to God's work, so Jesus urged his disciples not to wallow in unbelief but to recognise how God can use even a tiny amount of faith.  Our faith may seem small and insignificant (especially in times of hardship and testing), but as our faith grows, so does our potential to overcome great obstacles and achieve noble goals.

But why on earth would Jesus make that statement in reply to the apostles' request for greater faith?  Was he telling them that even the tiny amount of faith they had was enough for them to do great things, and not to under-value what they already had?  Or was he suggesting to them that, because they had failed to show faith, they obviously didn't even have faith as small as a mustard-seed?  I would like to think that his reason was the first one - a more positive and encouraging approach.  I would like to imagine that it was said with a twinkle in his eye and a loving grin on his face, so that the apostles would not feel put down or discouraged.

I pray that I might never give the impression to any struggling Christian brother or sister that their troubles may be due to a lack in their faith.  I don't believe Jesus would discourage anyone in that way.

Quest Study Bible note p 1449.  This is an example of  hyperbole, a figure of sppech not restricted to its literal meaning.  Rather than give his disciples a formula for increasing their faith, he pictured faith as a seed.  Plant a seed and it grows;  use a little faith and it will grow too.  We don't need to pray that God will increase our faith;  we need to ask him to help us use the faith we have.

V 7 - 10
'Suppose one of you has a servant ploughing or minding sheep.  When he comes back from the fields, will the master say, "Come along at once and sit down"?  Will he not rather say, "Prepare my supper, fasten your belt, and then wait on me while I have my meal;  you can have yours afterwards"?  Is he grateful to the servant for carrying out his orders?   So with you: when you have carried out all your orders, you should say, "We are servants and deserve no credit; we have only done our duty." '

Now, what has a servant or a slave doing his job got to do with faith?  This servant  had no expectation of praise from his master just because he was doing the job he was employed to do.  He was not expecting to be given a better job or a more responsible job or to have his wages increased just because he was getting on with his job and doing it properly. It wouldn't have hurt if the master did give a bit of praise or thanks now and again, but that would be a bonus - not a right to be expected.  In the same way, we are to do the work that God asks us to do without expecting to be given some kind of superior faith or to be rewarded in any special way.  Knowing that he trusts us with a task for him should be all the reward we need.  It takes trust or faith in a master to be able to work in this way.  The servant knew that, when his work was done, he would have time to sit down and eat his own meal - he trusted that the master would allow that and would proved the meal for him to eat.  We must be content to do God's will without asking for special privileges - such as more faith, more power, or to see results of our work.

Quest Study Bible note, p 1449: Jesus used an example familiar to the people from their own culture.  He wasn't saying that the master was right to be so unappreciative.  He was simply using to examle of a servant's attitude: a servant merely did what he had to do.  Jesus wants his disciples to see that strong faith should not lead to spiritual pride.  Our prayers should not be based on an attitude that says, "If I do this, then God will give me that."  God gives out of his grace, not as a reward for good behaviour.

New Bible Commentary p 1007.  (Jesus) is giving a necessary lesson to all who are tempted to feel proud of their faith or their good works for God.

Were the apostles, feeling that they were closer to Jesus than his other disciples, thinking that they should have extra 'hints' from Jesus about spiritual things and have more faith than others?  Was this part of their motive for asking to be shown how to have greater faith?