I would think "fraud" or "hypocrite" to be the most applicable words, barring a specific context. Note that the word hypocrite appears in common usage and is recognized by Oxford's but not Merriam Webster's dictionaries.
one who makes false claims of identity or expertise
A hypocritical person. (Oxford)
where Hypocrisy is:
the pretending of having virtues, principles, or beliefs that one in fact does not have (Merriam Webster)
For particular contexts:
If you are referring specifically to a person who acts in an inauthentic way intentionally for profit then the vernacular would probably be "con-man" or "con artist." This seems fitting since they trade on the confidence of their "mark" and that false confidence a con-man cultivates is generally a confidence of authenticity.
If, on the other hand, you are referring to an intangible or emotional status you might consider the term "impostor" which has seen a recent renaissance in the rise of impostor syndrome: essentially a doubting of one's own authenticity.
Synonyms of fraud
charlatan, fake, faker, hoaxer, humbug, impostor (or imposter), mountebank, phony (also phoney), pretender, quack, ringer, sham
Words Related to fraud
copycat, imitator, impersonator, mimic
actor, bluffer, counterfeiter, deceiver, dissembler, duper, feigner, misleader, operator, trickster
cozener, defrauder, dodger, scammer, sharper, sharpie (or sharpy), skinner, swindler
Synonyms of Hypocrite
sanctimonious person, pietist, whited sepulchre, plaster saint, humbug, pretender, deceiver, dissembler, impostor (Oxford)