Here’s an interesting bit from Dan Brown’s novel, The Lost Symbol: “Since the days of Michelangelo, sculptors had been hiding the flaws in their work by smearing hot wax into the cracks and then dabbing the wax with stone dust. The method was considered cheating, and therefore, any sculpture “without wax”—literally sine cera—was considered a “sincere” piece of art. The phrase stuck. To this day we still sign our letters “sincerely” as a promise that we have written “without wax” and that our words are true.”
This has been highlighted by 670 Kindle users (and counting), so I can only assume these people are taking Brown’s little literary flight of fancy seriously, as fact.
This is an example of Dan Brown’s wonderful imagination and inventiveness. But that’s not 
where the word “sincerely” came from. I mean, really, how do you go from carving sculptures to writing letters? While the word “sincere” most likely did come from “without wax,” it’s much more likely that the phrase originated from the idea that the writer, having written, is being so honest that s/he is not sealing the envelope.
Remember, they used to close envelopes with a wax seal. So a letter written and not sealed with wax is an “open letter,” for anyone to read. This implies no fear of someone refuting the honesty of what is contained within, thus it is “without wax” or sincere.
