Sincerity and Authenticity Are Not the Same
A person can be sincere and still be wrong about themselves. That is what makes sincerity and authenticity worth separating.
Sincerity concerns faithfulness to what we presently believe, feel, or intend. I can sincerely tell you my view. I can sincerely describe my motives. I can sincerely insist that I am being honest.
Authenticity asks an additional question. What if the account I am sincerely giving is itself incomplete? What am I not seeing about my performance, defensiveness, selective memory, or desire to look a certain way? This does not make authenticity a license for outsiders to declare that they know our inner truth better than we do. It makes self-examination part of honesty.
Sincerity says: this is what I currently mean. Authenticity can add: and I am willing to examine the places where my account does not match my conduct or where I may be hiding from myself. The distinction is uncomfortable. That is part of its value. Honesty about what we believe is important. Honesty about the limits of what we believe about ourselves may be harder.