I remember a story I once heard about the origin of the word "sincere".In Roman times, lacking power tools, polishing marble to a high luster, and hiding any blemishes in the marble, was a long labor-intensive task. Over time, a short cut was developed, whereby an artisan would cover the surface of the marble with wax making a luster easy to achieve.
Externally the wax could not be detected, until it was placed outside. A hot day would come...the wax would melt, the luster would disappear...all the blemishes on the surface of the marble could then be seen.
To remedy this deception, marble polished without wax would be stamped sine cera - "meaning without wax", "without deception". Obviously one would pay a premium price for a "sincere" or a pure piece of marble.
I can't think of a better way to illustrate the meaning of "being sincere".