Expositor's Bible Commentary Commentary Luke 17:5

"And the apostles said unto the Lord, Increase our faith." — Luke 17:5 (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.