What does Luke 17:6 mean?

Luke 17:1–10 is a collection of Jesus' sayings to His disciples. In the first passage, Jesus talks about the disciples' responsibilities regarding sin and the church. He ends by urging them to forgive the repentant, even if they sin several times in one day (Luke 17:1–4). Following, Jesus tells them they need to serve with extreme humility, not demanding any recompense (Luke 17:7–10).

Between these two self-sacrificing examples of leadership, Luke includes a teaching about faith. The disciples want more; Jesus says they can accomplish great deeds with what they've got.

Paul gives an example of this in a different context. When he asked God to remove the "thorn in his flesh," God told him, "My grace is sufficient for you, for my power is made perfect in weakness" (2 Corinthians 12:9). The disciples' faith is "weak." God doesn't care. He can work through weak faith. In his book The Reason for God: Belief in an Age of Skepticism, Tim Keller said, "It is not the strength of your faith but the object of your faith that actually saves you. Strong faith in a weak branch is fatally inferior to weak faith in a strong branch."

The plant mentioned here is a "fig-mulberry" or "sycamore," or "sycamine," labeled by a similar Greek word featured in a later story (Luke 19:4). These plants are grown for their fruit and can be quite tall with stiff branches. They have large root systems, making them hard to pull out from the ground. Such a plant obviously doesn't belong in the sea, nor would it accidentally get there. The difficulty of the act and absurdity of the image is meant to jar Jesus' listeners into understanding the depth of His meaning. The tense of "obey" means the tree was waiting for the command and ready to obey; the results are already assured.

On other occasions, Jesus made a similar comment using the imagery of ordering a mountain into the sea (Matthew 17:20; 21:21; Mark 11:23).