⁵ The apostles said to the Lord, “Increase our faith!” 

⁶ He replied, “If you have faith as small as a mustard seed, you can say to this mulberry tree, ‘Be uprooted and planted in the sea,’ and it will obey you. (Lk 17:5-6, NIV)

What kind of faith do we need? HOW MUCH faith do we need? I love NT Wright’s thoughts on this passage:

Perhaps not surprisingly, the disciples realize in verse 5 that all this will require more faith than they think they have. Jesus is quick to respond. It’s not great faith you need; it is faith in a great God. 
Wright, T. (2004). Luke for Everyone (p. 204). London: Society for Promoting Christian Knowledge.