Faith to Cast a Mulberry Bush into the Sea
Luke 17:5-10 is a teaching by Jesus on Faith. Verse 5 is actually a request by the disciples for Jesus to increase their faith. So, fasten you seat belts because Jesus is going to rocket into a deep teaching on increasing faith.
Verse 6 is more motivation then an explanation on how to increase our faith. In verse 6 he explains what can be achieved with even a small amount of faith.
Verses 7-10 are the meat of Jesus's teaching on how to increase your faith. As is usual he tells it using the symbolism of a parable. The good news is that he uses familiar symbolism. As you read through verses 7 through 10 you see two main characters, the master and the servant. The proper interpretation of the parable is that we are the servants and God is the master. He starts the parable out by saying if YOU have a servant. This gives you the heavenly view point of how God sees his servants. We see many places in Jesus teachings where he uses parables of master-servant or ruler-subject relationship. In such parables the master can usually be seen to be God and the servant to be us. Some examples are ... Need examples here.
Now that we are oriented with the actors, we need to interpret every statement as the" how to" of increaseing faith. Verse 7 talks about the servant coming in from the field "plowing or looking after the sheep". We recognize the symbolism of one plowing (ie planting seed or tending a crop) as the one spreading the Good News and preparing the way for a harvest of souls (eg see Chapter 2). The one tending the sheep we recognize as the shepherd. This is the work of the one tending to the children of God (eg see Chapter 1). So an essential step in increasing our faith is preparing for and reaping the harvest of souls and / or caring for God's lambs. It doesnt' say the servant is doing both of those tasks but he was busy doing one of those two essential tasks. I don't think this limits faith to the full time ministry, but it does imply that the servant is not sitting on his duff expecting God to jump at his bidding.
Verse 8 says that the servant must feed the master before he can expect to receive his sustenance. How do we feed God. Hopefully, chapter 3 on worship has made clear that the only way we can bless (ie "feed") God is with our devotion and worship. So, before we can expect to receive anything from the hand of God we must first worship him with a fullness of heart.
The other condition was the we must prepare ourselves. How do we prepare ourselves for entering in before a holy God with our petitions? If you read the chapter on holiness then the preparation should be clear. Note that the preparation must be done before we "serve" God. Only then can we receive from God.
The rest of the passage all has to do with humility.