When you have done all you have been commanded, say, "We are unprofitable servants; we have done what we were obliged to do" (Luke 17:10b).
One of the things that I sometimes do when trying to understand a particularly difficult Scripture passage (and I count today's Gospel passage as difficult) is to look at various translations. Verse ten is good case in point. While many different translations use "unprofitable" in rendering the Greek into English, other options include "worthless," "unworthy," "good-for-nothing," and "undeserving." However, the Weymouth New Testament translates the passage "There is no merit in our service." I think this is really what St. Luke is trying to say.
First of all, let us remember that St. Luke was a Gentile; and as such, he was part of the group that was used to being ignored by Jewish society. He was "far off" while the Jews thought of themselves as "drawn near." His Gospel bears this particular imprint as he includes so many instances of people who were considered second-class citizens. When he speaks of "unprofitable servants," he is referencing the many people in this society who were slaves. A slave who was either physically unable to serve his/her master was a "financial" disaster. Most of the translators find the word "unprofitable" particularly fitting.
However, St. Luke was probably heavily influenced by the Apostle to the Gentiles, St. Paul, and may even have heard St. Paul preach the Gospel. St. Paul's heavy emphasis on justification by faith also sheds some light on this passage, particularly in the Weymouth translation. Nothing we "do" can earn us a place in God's reign. We will never be able to buy our way into heaven with stored up merits for good deeds done. We come to salvation through faith in Jesus. Once we believe, then it is simply our duty to serve to the best of our ability. Faith "obliges" us to serve God; or, as St. James wrote, "faith without works is dead."
God will not count up the number of rosaries we recite, the good deeds we have done, the times we came to the aid of the poor when we come to judgment. God will simply ask if we believe and how that faith has led us to be true disciples, how that faith has manifested itself. Faith inspired lives of service as instruments of God are simply what is ours to do.