There was once a time when people aspired to be honest, truthful and sincere. This was prior to mass immigration and the consequential divisions in society. Since then, personal gain, spite and survival-motivated lies make it hard to be honest.
Oscar Wilde wrote, in "The Importance of Being Earnest", that in "matters of grave importance, style, not sincerity, is the vital thing".
Truth seems rarely to be what people seek. Wilde wrote, in "The Critic as Artist", to "know the truth one must imagine myriads of falsehoods. For what is truth?" Identifying the false is as hard as it is to identify the truth.
The media tells us so many lies, and successfully manipulates so many people, that the message coming through from this is that honesty is not always the best policy.
People tend to look at all the various philosophies, political ideologies and religions and choose which set of lies most appeals to them. Nietzsche said "Convictions are more dangerous enemies of the truth than lies".
Martin Luther said "Faith must trample underfoot all reason, sense, and understanding". Yet Martin Luther advocated faith. He was the father of Protestantism. The faithful can believe very sincerely in some kinds of lies.
It is not easy to tell if someone is being sincere or not. There is so much pretend sincerity, mainly in politicians, who tell you to trust them (something often said when they are tricking you!) and from the opinion-makers of the media, or pushy salespeople that sincerity itself has been devalued and the more sincerity that someone shows, the more you wonder if they are really just acting. But then perhaps most people don't really wonder this, and just fall for the trickery - then again we all tainted to some extent by this avalanche of false sincerity and so we practice it ourselves from time to time. Basically. we are living in an age of insincerity.