The word sin cera comes from the Latin words sine (without) and cera (wax). Back in the day, say oh, thousands of years ago, sculptors would use putty, or a wax, to fill in imperfections or cracks in a sculpture instead of working out the kinks. If a sculptor was true, he would do his work sin cera, without wax, making his work sincere (a deviation of the works sine and cera in Latin).
When we write letters and sign them 'Sincerely', it is likened to what the sculptors who were true in their work, symbolizing when we sign beneath that 'Sincerely' that our words are true and honest.
So if you still sign your letters and emails like this, do you stop and think to make sure your words are true?