There’s a logical fallacy called the “No True Scotsman fallacy” that I hear in play quite often. Here’s an example:
PERSON A: All Wisconsinites like cheese.
ME: I do not like cheese, and I am a resident of the state of Wisconsin.
PERSON A: Ah! But you are not a true Wisconsinite!
The No True Scotsman fallacy is when a person makes a hasty, ad hoc modification to a previous generalization when confronted with a counterexample that challenges their assertion.
Consider a very common example of this fallacy:
CHRISTIAN: Christians are better people than Muslims.
ME: Apartheid South Africa was very Christian, and I don’t think they were better people than Muslim groups.
CHRISTIAN: Well, they weren’t real Christians.
This sort of thinking is absolutely rampant, and it’s problematic, not only on a philosophical level, but also a political one. Who, exactly, gets to decide who the “True Christians” are? If you say “Well, I just believe in the Bible,” then I ask, whose Bible? Do you have the Apocrypha (Most protestants, no, Catholics/Orthodox, yes)? How do you interpret the text? Which translation? But more importantly, what does your liturgy look like? Do you include footwashing as a sacrament (as the Brethen Church does)? Are women allowed to speak at meetings (The Wisconsin Evangelical Lutheran Synod does not)? Which of the creeds do you accept? Do you believe in the possibility of universal reconciliation, or do you have to be born again to enter heaven? Do you have to speak in tongues to prove you are open to the Holy Spirit? Does the Book of Daniel/Revelation predict the future, or does it say something about the past? Did Jesus die for everybody, or just for God’s Elect?
I believe there is no essential property of Christianity, but a family resemble of faiths centered around Jesus of Nazareth. Thus, there is no point in telling others that another church is “Not True Christianity.” Rather, let us follow Jesus in showing that we can be known by our fruits.