What determines whether a person is a Christian or not? To what authority can anyone turn to verify a person’s or their own Christian authenticity?

Is every person who claims to be a Christian necessarily a Christian, even if they make no effort to follow Christ’s teaching? Does joining a Christian church make a person a Christian? Getting baptized? How about just attending, without joining?

Could a person who, through their own personal value system, happens to live very nearly the life Jesus recommended, be called a Christian even
if they don’t call themselves one?

Who gets to decide? The person, themselves? The church they attend? The church they don’t attend? Their neighbor? Some opinionated FB rando?

What makes a Christian a Christian?