Learn to weigh truth on truth scales
and weigh sincerity on sincerity scales.
– Jim Rohn

Sincerity is not equivalent to truth.  Just because you are sincere does not mean you are right.  You see, it is very possible to be sincerely wrong.  When you share what you
know with certainty, be certain that you really know what you are sharing.

“An often repeated folk etymology proposes that sincere is derived from the Latin sine = without, cera = wax. According to
one popular explanation, dishonest sculptors in Rome or Greece would cover flaws in their work with wax to deceive the viewer; therefore, a sculpture „without wax‟ would mean honesty in its perfection.[wiki]   The Oxford English Dictionary (OED), however, says there is no chance ‟sincerity‟ comes
from „sine cera.‟

Perhaps you have read The Professor and the Madman and are a bit suspect of some of the origins of the OED‟s definitions.  Just make sure your listeners are not entranced by your prose while you purloin their sense of reality.  When you make up a story and present it as the truth we call this lying.  When you repeat a story, which is not entirely accurate, you may be engaging in innocent  improvements or engrossing gossip.  Speaking with sincerity effectively, requires you commune with the truth, and not become seduced by the story.

Speaking truthfully is a weighty subject, but actually knowing “The Truth” remains beyond your reach.   While waxing eloquently can provide a memorable story of an ersatz etymology, the slightest research will leave your reputation wanting.

Your sincerity is experienced emotionally, the enduring value of your sincerity is its congruence with the truth. Sincere stories do not only sound true, they are.