The English word “sincere” comes from two Latin words meaning pure and without wax.
In ancient times, potters would sometimes have their pottery crack during the drying time in the heat. Some unscrupulous potters, rather than throw the flawed piece away, would fill in the crack with wax and paint over it.
A shrewd buyer, aware of this practice also knew that is you hold the finished piece up to the sun, the light from the sun would expose the flaws. The potter would cry, “sincere, sincere!“ Meaning, “without wax.” If you want God to deliver you from the fear of man you must be “sincere!“