I collect quotes about ‘faith” - here is my first: "The easy confidence with which I know another man's religion is folly teaches me to suspect that my own is also." Mark Twain (1835-1910)
If I’m talking to a Christian I use Hebrews 11:1: “Now faith is the assurance of things hoped for, the conviction of things not seen.” - Hope in the invisible, that’s a pretty shitty ideal to dedicate one’s life to.
Matt Dillahunty has a good definition: Faith is the excuse people give for believing something when they don't have a good reason.