The strength of a man's virtue should not be measured by his special exertions, but by his habitual acts.Blaise Pascal — Pensées, 1670
Sincerity is the alignment between inner state and outer expression — saying what you mean, meaning what you say, and acting in accordance with what you genuinely believe rather than what you calculate is expected or rewarded. It is the quality that makes trust possible, because trust depends on the prediction that what a person says corresponds to what they actually think and will do.
Confucius regarded it as essential to the superior person: daily self-examination on whether one had been sincere in all transactions was the basic practice of moral cultivation. Not whether one had been successful or admired — whether one had been true. The distinction matters because success and admiration can be obtained through performance of sincerity without its substance. The performance, over time, corrupts the performer.
Dale Carnegie understood sincerity as the foundation of genuine human relations rather than the manipulation of them. The famous observation that you can make more friends in two months by becoming genuinely interested in other people than in two years by trying to get others interested in you is an observation about sincerity: performed interest fails; genuine interest succeeds. The difference is detectable, over time, by almost everyone, because people are better calibrated detectors of authenticity than most social performers realize.
Sincerity does not mean impulsive self-disclosure. It means that what you say, you believe, and that what you believe, you act from. The person who gives polite agreement they do not feel, who praises what they privately dismiss, who promises what they have no intention of delivering, is not being socially skilled. They are being false, and the falseness produces consequences — in the erosion of their own integrity, in the eventual breakdown of trust — that are more costly than the momentary awkwardness of honesty would have been.