And when he (Barnabus) had found him (Saul of Tarsus), he brought him unto Antioch. And it came to pass, that a whole year they assembled themselves with the church, and taught much people.
And the disciples were called Christians first in Antioch. Acts 11:26.
Initially, in Antioch, there was a company of people taught by Barnabus and Saul.
They received certain teaching, which they all believed, and they all conformed to certain behaviour, such that other people, round about them, saw a distinction in this specific gathering and called them 'Christ-ian'.
They believed in Christ and they followed Christ as Christ was represented to them by certain leaders.
Many now self-identify themselves as 'Christ-ian' but they have not received the teaching that Saul (now known as Paul) expressed in his writings. They have a bible, but they do not actually receive what is written therein.
Paul himself, in his writings, predicted this would happen. It was already evident within his own lifetime.
The 'Christianity' site on Stack Exchange invites questions regarding all of that which identifies itself as 'Christian' and the site acknowledges the diversity of what now exists.
It remains to the individual to receive what they are led to receive and to reject what they are led to reject.
The secret is in the 'leading'.
Jesus said 'My sheep hear my voice and they follow me'. Some hear that voice and follow in the footsteps of the true flock. Some hear another voice (much as Eve heard a voice when she was in the garden of Eden) and they are led astray.
As Jesus remarked himself, it is not a matter of 'Lo, here !' or 'Lo, there !' (externally to oneself) but :
Behold, the kingdom of God is within you [Luke 17:21, KJV]
. . . or, conversely, it is not.
So, many will self-identify, but few are chosen. [cf Matthew 22:14, KJV]