Perhaps there is no other knowing than the mere competence of the act. If at the heart of one's being, there is no self to which one ought to be true, then sincerity is simply nerve; it lies in the unabashed vigor of the pretense. But pretense is only pretense when it is assumed that the act is not true to the agent. Find the agent.
Because a thing is eloquently expressed it should not be taken to be as necessarily true; nor because it is uttered with stammering lips should it be supposed false.
By this time, like one who had set out on his way by night, and travelled through a region of smooth or idle dreams, our history now arrives on the confines, where daylight and truth meet us with a clear dawn, representing to our view, though at a far distance, true colours and shapes.
The first law for the historian is that he shall never dare utter an untruth. The second is that he shall suppress nothing that is true. Moreover, there shall be no suspicion of partiality in his writing, or of malice.
The dangers is that every religion, including the Catholic one, says "I have the ultimate truth." Then you start to rely on the priest, the mullah, the rabbi, or whoever, to be responsible for your acts. In fact, you are the only one who is responsible.
Truth is compared in Scripture to a streaming fountain; if her waters flow not in perpetual progression, they sicken into a muddy pool of conformity and tradition.