It can, as I pointed out recently, refer to the practice of highlighting and even exaggerating personal flaws for the sake of connecting with people, owing to a cultural moment in which the fragile and the ordinary are prized above the robust and the exemplary. That, I contended, is a fad, and needs to be resisted by those who have the privilege to serve churches by leading and teaching them.
On the other hand, it can (and often does) refer to something overwhelmingly positive, whereby the leader’s life - strengths, weaknesses, family life, spiritual disciplines, personal evangelism - is so visible to all, and so accessible to all, that the church can see what Christian living looks like in reality, and not just in theory. In a culture like ours, where much of life (even in the church) takes place behind closed doors, and where it is possible to manage carefully the perceptions people have of us by controlling which aspects of our lives we want them to see, this sort of authenticity is vitally important and radically countercultural. We have a young man living with us at the moment, as a family, and one of the benefits of doing this has been having our whole lives - not just the antiseptic bits - on display for him to see. He may or may not desire to emulate what he sees, but if he does, he will be imitating the reality rather than a stage-managed adaptation of it.
This second sort of authenticity, in contrast to the first (often, ironically, self-promoting) sort, is what Paul, Silas and Timothy modelled in Thessalonica:
You know what kind of men we proved to be among you for your sake. And you became imitators of us and of the Lord… (1 Thess 1:5-6)
So, being affectionately desirous of you, we were ready to share with you not only the gospel of God but also our own selves, because you had become very dear to us. For you remember, brothers, our labour and toil: we worked night and day, that we might not be a burden to any of you, while we proclaimed to you the gospel of God. You are witnesses, and God also, how holy and righteous and blameless was our conduct toward you believers. For you know how, like a father with his children, we exhorted each one of you and encouraged you and charged you to walk in a manner worthy of God, who calls you into his own kingdom and glory. (2:8-12)
In other words, the apostles were thoroughly authentic - not by emphasising or exaggerating their weaknesses, but by living their lives in plain view of the church. And that is something of which the global church needs much more, not less.
So yes, being authentic could, if understood and practised wrongly, be simply a fad. But if understood and practised correctly, it has the power to put the gospel on display in a way that little else does.