(The following questions come from my brief guide to Law and Gospel preaching, Afflicting the Comfortable, Comforting the Afflicted, available from wipfandstock.com or amazon. They are meant to get at some of the key questions for preachers of this genre. For more insight into this type of preaching, check out my guide.)
1. How does the Word function in the text? Because of the construction of the saying by Jesus in verse 6, the Word functions here as gospel. Jesus is saying, in effect, "The faith you have been given is sufficient; celebrate that! The faith you have, though small, is more than enough to do all that you dream of and more."
2. How is the Word not functioning in the text? Again, because of the construction of the saying, the Word is not functioning here as Law. It is not saying, "If only you had a wee bit of faith, you faithless ones, then you could do great things." No, because faith is a gift, and because God's grace is sufficient, even the tiniest piece of faith, like the tiniest sip of wine or taste of bread at the Lords' table, is sufficient for our needs.
3. With whom are you identifying in the text? We are undoubtedly the apostles who say to the Lord, "Increase our faith!" We wish to have greater faith because we mistakenly believe that more faith will mean greater power or greater wisdom, or perhaps even a greater station in the household of our Lord. The parable which follows Jesus' saying shows clearly that even when we have done all things well, we are simply the servants of Christ, no better or worse than any other servants in the household. So with faith - greater faith will not elevate us to a different place in the Lord's household.
4. What, if any, call to obedience, is there in this text? The parable within this text is simply that - a call to obedience. We are to do all that our Lord commands us, and when we have done that we have only done what is rightfully our responsibility. We are servants; that is our identity.
5. What Law/Gospel couplet is suggested by this text? Several obvious couplets come to mind in this text: discontent with faith/trusting God; discontent with our station/trusting our Lord.
6. Exegetical work: There is an extensive article about faith in Kittel's Theological Dictionary of the New Testament. I would recommend it to you. Here are a few pieces I found especially helpful: "As the Old Testament understands it, faith always is man's reaction to God's primary action. Related here is the fact that older Old Testament religion was collective in structure, and it was difficult to give expression to the inner life of the community." (TDNT, VI, 182) "'Faith' in the Old Testament denotes a relation to God which embraces the whole man in the totality of his external conduct and inner life." (TDNT, VI, 188) General Christian usage of the word faith is 1) To believe God's word; 2) To obey; 3) To trust (i.e. God will fulfill promises); 4) To hope; 5) To be faithful. (TDNT, VI, 205) Martin Luther had this to say about faith in his Sacrament of Penance, 1519: "Now if God allows faith to remain weak, one should not despair on that account, but rather recognize it as a trial and temptation by means of which God tests, prods, and drives a person to cry out all the more... with the apostles, "O Lord, increase our faith." Thus does a person come to learn that everything depends on the grace of God; the sacrament, the forgiveness, and the faith. Giving up all other hope, despairing of himself, he comes to hope exclusively in the grace of God and cling to it without ceasing." (Luther's Works, 35:19).
7. Consider the insights of the pioneers of the New Homiletic? Henry Mitchell always urged preachers to include celebration in their sermon design. Since this text lifts up the sufficiency of even a wee bit of faith given to us by God, what better time to celebrate than that?
Blessings on your proclamation!