John Lightfoot on Luke 17:6

[As a grain of mustard seed.] A phrase greatly in use. Sometimes we have it like a seed of mustard. Sometimes, like a grain of mustard seed. Sometimes, like a drop of mustard.

When our Lord had been teaching his disciples concerning charity towards their offending brother, they beg of him increase our faith. Which words (saving that I would not wrong the faith of the apostles, as if they begged of their Master an increase of it) I would inquire whether they might not be put into some such sense as this: "Lay down or add something concerning the measure of our faith, as thou hast done concerning the measure of our charity": which, therefore, he doth in his following discourse.