Expositor's Bible Commentary Commentary Luke 17:6

"And the Lord said, If ye had faith as a grain of mustard seed, ye would say unto this sycamine tree, Be thou rooted up, and be thou planted in the sea; and it would obey you." — Luke 17:6 (ASV)

The apostles may have felt that this kind of forgiveness would demand more faith than they had. The black mulberry tree grew quite large, to a height of some thirty-five feet, and would be difficult to uproot. The mustard seed is proverbially small, a suitable metaphor for the amount of faith needed to do the seemingly impossible (Mark 11:23). Jesus’ answer to the request for additional faith seems to be that they should use the faith they had to petition God.