Stones in general represent true ideas. Flint is a kind of stone that can be chipped to form extremely sharp edges. To keep the metaphor going we would want flint to be truth that "comes to a point", and in a way that's exactly what it is. According to the Writings, flint represents the "truths of faith" the rules we learn about how to be good people and serve others so that we can apply them as they arise in life. These are relatively black-and-white concepts ("Thou shalt not murder") that help us identify evil desires and arm ourselves against them. This is why flint knives were (and in some cases still are) used for circumcision, which represents purification from our most base love of self and love of the world -- something that must be accomplished by truths of faith.