Jesus' words in St. Luke 17:1-10 present us with two religions. One is a religion of laws and rewards, the false religion of the Pharisees who were expecting rewards for their deeds. The other is the religion of mercy, by which we learn to forgive someone who sins against us even up to seven times in a day. The apostles said, "Increase our faith," and Jesus' reply was this: "If ye had faith as a grain of mustard seed, ye might say unto this sycamine tree, Be thou plucked up by the root, and be thou planted in the sea; and it should obey you."
What does this mean? Perhaps this is a reference to the false religion, coming up from generation to generation (like the roots of a tree), which leads all the way back to Eden and the tree of the knowledge of good and evil. From this tree springs the religion of laws and works, and its end is death. It must be planted in the sea, that is, drowned in Holy Baptism, that a new tree (a new religion) of mercy might arise, the tree of life and salvation.
The sermon is here.