As grown-up people, we need to overcome groupishness and passive acceptance of external sources of authority - and this applies to Christianity as much as anything else.
At present, many/ most people regard it as a satisfactory evaluation analysis of Christianity to encounter somebody self-described as A Christian who does something abhorrent to the evaluator; and then to deploy some version of the phrase: If that is Christianity - then I want nothing to do with it!
Yet on that basis, there is nothing-at-all which could be regarded as Good.
By saying 'if that is Christianity...', all we are deciding is that we do not want to be that. But we are not correct in assuming that that is indeed real Christianity, merely because somebody-else says-so...
To know the reality of a situation, we cannot evaluate by group categories, neither as an average nor as the worst member of a group - we must go behind the group and into our-selves. And in going behind the group, the evaluation must come from our-selves.
And in evaluating from our-selves, the evaluation needs to be of the heart, of the intuition of the real self - because it is useless to deploy superficial and socialised evaluations because they are mutually-contradictory, labile and manipulable.
We must also go beneath the surface of our multiple false selves. Only when we can get down to the level of our the real-self evaluating the essence of the situation, can we attain the stability and validity that is pre-requisite to truth; attain to something we can build-upon.
Behind the group, beneath the surface... this clarifies why the general level of normal public discourse is absolutely worthless to us if we really want to evaluate the truth of anything - and this constraint naturally applies to evaluating Christianity.
Nobody-else and nothing-else can do this for us; so if we really want it done, we must do it our-selves. We can do it ourselves, but doing it absolutely entails stepping outwith the current and prevailing ways of thinking and being, the usual conventions of what counts as true.
And this means that we will not be able to justify our evaluations to other people, not be able convincingly to explain our reasoning - or at least not to people operating in the mainstream of social discourse; and especially not to those in leadership positions (i.e. those whose role is to monitor and implement the current, prevailing, usual ways of thinking and behaving).
In conclusion, we can know-for-our-selves the truth and validity of Christianity, or of other vital matters; but only by stepping outside the mainstream ways of discoursing.
To those remaining on the inside, who have assimilated and endorse the assumptions of the mainstream, we will seem to be stupid or irrational - or simply dishonest and manipulative.
Yet if we really want to know the real truth, Christians will need to expect and accept this Outsider status.