Who is "truly" a Christian is a controversy that has roots all the way back to the days of the Christian Testament, an assemblage of writings dating from about the year 45 through around the year 110. Both the gospels of Mark and Luke recount episodes where the apostles approach Jesus to report seeing people they didn't know acting in Jesus' name. The apostles are concerned that these people they don't know aren't true Christians.
On several occasions, Paul writes to the congregations he's founded and tried to stop them from drifting in their beliefs and practices. For Paul, the only true Christianity is what he has been teaching (cf. the letter to the Galatians). This is an old, old question, and I don't really have a satisfactory answer. All kinds of people use the same words, but they really have different meanings, depending on the person, the context, each person's experience, and a bunch of other factors. It leads to a lot of misunderstandings and hostilities because, for example, both persons are talking about "what the Bible says," but each one has a different exegetical method, lends greater or lesser credence to passages and verses, and comes to a conclusion that the other person regards as unsupported.
What I have come up with for myself is "the Hawkeye dodge." In the first Avengers movie, as Hawkeye and Poison Ivy are working their little corner of the final battle against the invading aliens, Poison Ivy hollers over to Hawkeye that "This is just like Budapest all over again." Hawkeye, preoccupied with shooting arrows at the invaders replies, "You and I remember Budapest very differently." When I'm in a discussion with someone about the Bible, I have gone to the dodge of "You and I read the Bible very differently." I'm not saying I'm right and the other person is wrong, but that we read, interpret, and experience these texts quite differently.
It hasn't yet, but I'm hopeful that one day the Hawkeye dodge turns the conversation to, "Well, how do YOU read this passage?" or "Why don't you think my reading is the one, only, true, and correct reading?" What I've mostly discovered is that a deeper dive isn't what a lot of Christians want; they have their well-worn passages and verses, they know exactly what they mean, and further discussion is of the devil or something equally dire. Loud Christians who extol the virtues of Trump read the Bible very differently from me.
= new reply since forum marked as read
