The apostles said to the Lord, "Increase our faith!" 6 And the Lord said, "If you had faith like a mustard seed, you would say to this mulberry tree, 'Be uprooted and be planted in the sea'; and it would obey you. 7 "Which of you, having a slave plowing or tending sheep, will say to him when he has come in from the field, 'Come immediately and sit down to eat'? 8 "But will he not say to him, 'Prepare something for me to eat, and properly clothe yourself and serve me while I eat and drink; and afterward you may eat and drink'? 9 "He does not thank the slave because he did the things which were commanded, does he? 10 "So you too, when you do all the things which are commanded you, say, 'We are unworthy slaves; we have done only that which we ought to have done.'"
In Luke 17:5 the apostles ask Jesus to increase their faith. How does Jesus help them? In two ways, both of which are by telling them truth. So even in the way he responds he shows us that faith comes by hearing. Knowing certain things should increase our faith.
First, he strengthens our faith by telling us in verse 6 that the crucial issue in accomplishing great things to advance the kingdom of God is not the quantity of our faith, but the power of God. He says, "If you had faith like a mustard seed, you would say to this mulberry tree, 'Be uprooted and be planted in the sea'; and it would obey you." By referring to the tiny mustard seed after being asked about increased faith, he deflects attention away from the quantity of faith to the object of faith. God moves mulberry trees. And it does not depend decisively on the quantity of our faith, but on his power and wisdom and love. In knowing this we are helped not to worry about our faith and are inspired to trust God's free initiative and power.
Second, he helps their faith grow by telling them in verses 7-10 that when they have done all they are commanded to do, they are still radically dependent on grace. Jesus gives an illustration. You might want to read it again in verses 7-10. The gist of it is that the owner of a slave does not become a debtor to the slave no matter how much work the slave does. The meaning is that God is never our debtor. Verse 10 sums it up: "So you too, when you do all the things which are commanded you, say, 'We are unworthy slaves; we have done only that which we ought to have done.'" We are always his debtor. And we will never be able to pay this debt, nor are we ever meant to. We will always be dependent on grace. We will never work our way up out of debt to a place where God is in our debt. "Who has ever given a gift to him that he should be repaid?" (Romans 11:35).
When it says in verse 9 that the owner does not "thank" the slave, the idiom for "thank" is provocative. I think the idea is that "thanks" is a response to grace. The reason the owner does not thank the slave is that the servant is not giving the owner more than what the owner deserves. He is not treating the owner with grace. Grace is being treated better than you deserve. So it is with us in relation to God. We never treat God with grace. We never give him more than he deserves. Which means that he never owes us thanks. God never says "Thank you" to us. Instead he is always giving us more than what we deserve and we are always owing him thanks.
So the lesson for us is that when we have done all we should do – when we have solved all our pastoral care problems and fixed the attitudes of all our people and mobilized the most missions and loved the poor and saved marriages and reared godly children and boldly proclaimed Christ – God owes us no thanks. Instead we will at that moment relate to him as debtors to grace just as we do now.
This is a great encouragement to faith. Why? Because it means that God is just as free to bless us before we get our act together as he is after. Since we are "unworthy" slaves before we have done what we should, and "unworthy" slaves afterwards as well, it is only grace that would prompt God to help us. Therefore he is free to help us before and after. This is a great incentive to trust him for help when we feel like our act is not together.
So two things increase our faith: 1) that God himself and not the quantity of our faith is the decisive factor in flinging mulberry trees out of the way; and 2) free grace is decisive in how God treats us before and after we have done all we ought to do. We never move beyond the need for grace. Therefore let us trust God for great things in our little faith, and let us not be paralyzed by what is left to be done in our lives and in our church.