Lord, increase our faith…One should not blame the disciples for requesting Jesus to ‘increase’ their faith. What they had just heard from Jesus sounded undoable if not outright impossible. They were familiar with the Mosaic ‘ eye for an eye’ teaching (cf. Matthew 5:38; Exodus 21:24). They very well knew that one was to be given the same treatment as he/she had shown others (cf. Matthew 7:12). Yet, here was Jesus telling them that the law of the kingdom of God demanded that they surpass the expectation of the old law (cf. Matthew 5:20, 48b). And even if they agreed to give this new teaching a try, forgiving the same person seven times in a day was to ask too much of them. To make matters worse, they were now being told that one could be held responsible for the sin of another. This was taking the ‘little’ faith they had a little too far. 
In his response, Jesus reminds them (and us) that there is nothing like ‘little’ or ‘great’ faith. An individual is either a man/woman of faith or he/she is not. Faith is not a commodity that can be quantified (and just in case it can, faith the size of a mustard seed would be enough to do wonders). Faith is not something that is extraneous to an individual. Faith is demonstrated in how an individual lives his/her life (faith is lived). In other words, the disciples needed not to worry about the seeming impossibility of the life required of them, for as a people of faith, impossibility ceased being an option the moment they said YES to Jesus.