The origin of the word for sincerity is the Latin ‘sine cera’ meaning 'without wax'. At the time of Ancient Rome statues were in vogue.  Roman villas would be aligned with statues in the same way that modern gardens are filled with garden gnomes. 

Sculpture was a popular trade. The talented sculptors were artists.  The poor sculptors would correct their faults with wax.  The problem was that once the statue was brought home, the wax would run and reveal the imperfections.  Thus, there were statues 'without wax' and with wax; sincere statues and insincere statues. 

In the modern world where there are more design and development engineers than sculptors, there are still sincere products and insincere products.

Or, alternatively, 'sine cera' symbolized a document that contained no seal - i.e. no wax - when in lieu of a signed and sealed contract document, a shake of the hand and an honest eye was enough to guarantee honour, trust and integrity.  

I don't know which is true (I suspect the second), but I like both stories.