Increase our faith - This duty of forgiving offences seemed so difficult to the disciples that they strongly felt the need of an increase of faith. They felt that they were prone themselves to harbor resentments, and that it required an additional increase of true religion to enable them to comply with the requirements of Jesus. We may learn from this:

1. That Jesus has "the power" of increasing the faith of his people. Strength comes from him, and especially strength to believe the gospel. Hence, he is called the "Author and Finisher" of our faith, Hebrews 12:2.

2. The duty of forgiving offences is one of the most difficult duties of the Christian religion. It is so contrary to our natural feelings; it implies such elevation above the petty feelings of malice and revenge, and is so contrary to the received maxims of the world, which teach us to "cherish" rather than to forgive the memory of offences, that it is no wonder our Saviour dwells much on this duty, and so strenuously insists on it in order to our having evidence that our hearts have been changed.

Some have thought that this prayer that he would increase their faith refers to the power of working miracles, and especially to the case recorded in Matthew 17:16-20.

Luke 17:5 Parallel Commentaries

Thankfulness for Mercies Received, a Necessary Duty
Numberless marks does man bear in his soul, that he is fallen and estranged from God; but nothing gives a greater proof thereof, than that backwardness, which every one finds within himself, to the duty of praise and thanksgiving. When God placed the first man in paradise, his soul no doubt was so filled with a sense of the riches of the divine love, that he was continually employing that breath of life, which the Almighty had not long before breathed into him, in blessing and magnifying that all-bountiful, …
George Whitefield—Selected Sermons of George Whitefield

Not with Outward Show
Some of the Pharisees had come to Jesus demanding "when the kingdom of God should come." More than three years had passed since John the Baptist gave the message that like a trumpet call had sounded through the land, "The kingdom of heaven is at hand." Matt. 3:2. And as yet these Pharisees saw no indication of the establishment of the kingdom. Many of those who rejected John, and at every step had opposed Jesus, were insinuating that His mission had failed. Jesus answered, "The kingdom of God cometh …
Ellen Gould White—The Desire of Ages

Journey to Jerusalem. Ten Lepers. Concerning the Kingdom.
(Borders of Samaria and Galilee.) ^C Luke XVII. 11-37. ^c 11 And it came to pass, as they were on their way to Jerusalem, that he was passing along the borders of Samaria and Galilee. [If our chronology is correct, Jesus passed northward from Ephraim about forty miles, crossing Samaria (here mentioned first), and coming to the border of Galilee. He then turned eastward along that border down the wady Bethshean which separates the two provinces, and crossed the Jordan into Peræa, where we soon …
J. W. McGarvey—The Four-Fold Gospel