Jesus has shown that the master expects service first (Luke 17:8). Now He asks whether the master owes the servant thanks for simply doing what he was told. He answers His own question at once: "I trow not," which Barnes explains as I "think" not, or I "suppose" not. The question prepares the apostles for the point Jesus is making: their obedience does not put God in their debt. Matthew Henry sums it up: "Christ's servants do not so much as merit his thanks for any service they do him." Gill likewise says the servant only did what, as a servant, he ought to do.

Jesus answers His own question: "I trow not" (Luke 17:9). Then He applies it: "So likewise ye, when ye shall have done all those things which are commanded you, say, We are unprofitable servants: we have done that which was our duty to do" (Luke 17:10). The apostles give no recorded reply. The next scene, on the way to Jerusalem, is the cleansing of ten lepers (Luke 17:11-19).

"Trow" is an old English word for think or suppose, as Barnes notes. The phrase "I trow not" stands in the Textus Receptus, the Greek text behind the KJV (ou doko, "I think not"). The Greek texts behind most modern versions, such as the Nestle and Westcott-Hort editions, do not have it. That is why the ESV ends the verse with the question.

In the Greek, the master's "thanks" is the word charin, given to a servant for doing "the things that were commanded." A servant who did his assigned work was doing his duty, not giving a gift.

Jesus is describing how His servants should see their own work, not how God feels about them. The parable sets up Luke 17:10: when we have done everything commanded, we say we are "unprofitable servants." Barnes gives reasons: our service does not add to God, the grace to do His will comes from Him, and "Our best services are mingled with imperfections." Matthew Henry adds, "No good works of ours can merit any thing at the hand of God."

The very next story turns the idea around. Ten lepers are healed, and Jesus asks, "where are the nine?" (Luke 17:17). The master in the parable owes no thanks, yet those who receive mercy from God owe Him thanks. Grace runs from God to us, and gratitude runs back.