A useful, simple, pragmatic definition of sincerity:
"Saying on the outside what's happening on the inside"
... so that when you ask, promise, declare or inquire those who are listening are offered the possibility of trusting what you have to say... so that when you speak you are also offered the possibility of trusting what you have to say... and so we don't have to spend so much of our time and energy playing games - trying to figure out what each other want, and need, and are really committed to.