The commonly simple (WRONG) definition
A simple definition that is commonly given and understood by people of “what is a true Christian?” is simply, “a Christian is what I am.” Although this is understandable, it is not biblical.
At this point, we have to make some very clear distinctions from Scripture here as to what is, and what is not a true Christian. Any religious group has to be judged by some criteria, and we always want to return to our spiritual authority (The Holy Scriptures) to define what is what. No group “is Christian” just because. The group is a human, earthly manifestation of a group of people gathering around some religious ideas.
(to be developed in the future)
Criteria for determining what a religious group “is” essentially
So many religious groups practice what is called “bait-and-switch” where they proclaim one thing, and then actually do or are another. This is common in false religions because their underlying basis of operation is to deceive, so they think nothing of doing it.
A careful and cautious Christian has to probe beyond the exterior to see what the group really is.
Defining what is true salvation
True salvation is the heart of any religious group (if it is biblical). So many religious groups and churches define “being one of them” as loyalty to the group, not true salvation. People are in the group that do not express the characteristics of a truly saved person, and that doesn’t bother the group members nor the group leadership
The “mixed nature” of any true Christianity
This is exactly what Roman Catholicism has been over all the years it has existed. It has twisted and trans-morphed many religious elements into something it wants to identify as “Christianity”. Over the course of the Old Testament, for example, we see a constant denouncing and chastising of Israel for making images and then including them in their worship (whether they worshiped the image or not, it was a prominent element of worship). This is amazing that with this history, the Roman Catholic religion has made images so prominent in their worship.