Luke 17: 5-10
The apostles said to the Lord, "Increase our faith." The Lord replied, "If you had faith the size of a mustard seed, you could say to this mulberry tree, 'Be uprooted and planted in the sea,' and it would obey you. Who among you would say to your slave who has just come in from plowing or tending sheep in the field, 'Come here at once and take your place at the table?' Would you not, rather, say to him, 'Prepare supper for me, put on your apron and serve me while I eat and drink'? Do you thank the slave for doing what was commanded of him? So you also, when you have done all that you were ordered to do, say, 'We are worthless slaves; we have done only what we ought to have done.'" (Translated and paraphrased by Harry T. Cook.)

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This passage has never felt good or right, not to this exegete. For one thing, it seems to constitute an insertion of some kind into the larger text. Abruptly the conversation switches from being between Jesus and his disciples to being between "The Lord" and the "apostles." When the word "Lord" appears in a New Testament text in the way it does in the particular one now under consideration, it usually refers to the inexplicable presence of the dead Jesus among living people. The use of the Greek term αποστολοι is a clear indication of what is long since obvious, viz. that Luke and all the other gospels, with the possible exception of Thomas, are post-70 C.E. The fictional account of how "disciples" (followers) became "apostles" (commissioned leaders) is creatively spelled out in Acts 2 composed near the end of the first century or, by some accounts, well into the second century as the bare outlines of the Christian church were beginning to appear.

Luke 17: 5-10 includes two disparate sections, the first a colloquy between the "apostles" and "the Lord"; the second a series of rhetorical questions presumably addressed to the apostles (if they are the "you" of v. 7). The only thematic thread, if there be one, is obedience to a command. The idea seems to be that if the command is based on faith it will be done. That might put a whole new spin on the well-known petition "Thy will be done on Earth as it is in heaven," requiring the trust, loyalty and courage which together constitute the definition of "faith."
 
The petition for an "increase" (προσθες) of faith could be a response to what appears one verse earlier than the beginning of this passage in which Jesus is depicted as chartering the culture of forgiveness. Forgiveness is not easy. It takes trust in oneself, loyalty to the value of love and the courage of humility. One might appreciate their being "increased." Προσθες means "add to," as in a prosthesis.
 
One supposes that the fabric of faith (as defined above) was wearing rather thin for Jesus Jews at the end of the first century. The promised parousia had not materialized, and it probably had not gotten any easier to be a follower of Jesus Judaism in the face of hostility on the one hand from Rome and on the other from the synagogue. So "increase our faith" could have become almost a liturgical petition by the time Luke became aware of Matthew 17:20 in which "little faith" appears, as does the image of the mustard seed. Matthew would move a mountain from one place to another. Luke, as we have seen, would merely transplant a tree from soil to sea. If one had even a bit of the combination of trust, loyalty and courage, he could make big things happen, is the thrust of the passage.
 
A trouble arises in the later verses in which the concept of slavery is introduced. The whole of the passage is hyperbole, nevertheless it comes across as pretty rough. Certainly the house slave of a first century Palestinian household was largely unseen as long as he or she did the tasks proper to his or her slavehood. The slave did not expect thanks, only the security of upkeep.
 
The point, rather overdone it seems to me, is that a person is to fulfill the expectations that life and circumstances put upon him. The disciple follows; the apostle carries the message. Neither is to expect extraordinary praise or commendation for doing do. Such is their work. Such a point might as easily have been made without the introduction of the slave image.

If you work in an office and are paid for showing up at 8:30 a.m. and, with an ample lunch period, are expected to remain until 5 p.m., all the while fulfilling your appointed tasks to the best of your ability, you do not ordinarily expect thanks. Your paycheck, which is not thanks, is compensation for your time and energy.
 
If you are a member of a religious community that has well-defined responsibilities for its constituents, you should not expect to be thanked or fawned over for doing what is the duty you signed on to perform.
 
If your trust in your community is firm, if your loyalty to its mission is unquestioned and you are possessed by the courage of those convictions, what can you not do or at least try mightily to do to further the cause of your community's purposes, aims and goals? Why should you wait around to be noticed and commended for doing that which you have pledged to do?