People say they are part of groups all the time, but aren't. They might be mistaken, or they might just want others to think they are part of that group.

This sort of thing happens all the time in all parts of life.  I could use dozens of other examples to show how this works for everything else in life, but let's just use this non-controversial pair:

Consider for a moment: people who are hypochondriacs, and people with Munchausen (also known as "factitious disorder"). 

Hypochondriacs are people who sincerely (but incorrectly) believe they are sick with a serious disease, but demonstrably do not have that sickness. Hypochondriacs maintain their belief that they are sick, even when medical professionals are able to demonstrate that no illness is present. 

People with Munchausen, however, know that they are not sick, but do things to their own bodies to appear to be sick. They might give themselves mild poisons so that they vomit, or put chemicals on their skin to create a rash.  These people want others to think that they are sick.

So when it comes to Christianity, it's sort of the same thing.

Much like a hypochondriac (who is very bad at assessing their own condition), someone might think they are a Christian for any number of (wrong) reasons. Like, "my parents were Christians, so I am a Christian," or "I go to a church, so I am a Christian," or my personal least-favorite, "I say I am a Christian, so I am a Christian."  That would be like saying "I say I am a member of Mensa so I am a member of Mensa."  That's not how that works. It's not your club, and the entry requirement isn't just claiming membership.

Also, much like someone with Munchausen's, someone might just want other people to think they are a Christian, for any number of (stupid and evil) reasons. Usually those reasons are just to have some kind of admiration or influence over other people, or even just for money.  Jesus used the word "hypocrite" to describe this sort of person, and the stuff he says to them is pretty chilling when you read it. The word "hypocrite" literally means "actor," which is a pretty great word to explain the sort of pretending that these people do. What's truly terrible is when someone has been acting for so long, they can't tell they are acting anymore.

Jesus specifically said that there would always be many mistaken-or-pretend Christians. If you want to see a few references for that, check out Matthew 7:21-23, Mark 4:13-20, and Luke 13:23-24. Jesus said this sort of thing a lot; it's hard to read the words of Jesus and not run into his warnings about fake Christians.

The most-heard and least-understood phrase that the Bible uses is probably "believing in Jesus." Unfortunately, in America today, people us the phrase "believe in" to mean "agree with" or just "acknowledge that a thing exists," which is not at all how anyone was using that phrase back in the first century (when the New Testament was written).  Simply acknowledging the truth of some facts doesn't help anyone. James (one of the half-brothers of Jesus) even wrote a letter, addressing a group of Jews who were having trouble with this concept. In James 2:19, he writes, "You believe that there's one God. Good for you. The devils also believe that...and tremble." (JPV)

Believing that a thing is true is different from believing in that thing. 

One more example of this distinction, and hopefully that is all that we will need:

Imagine you are in a waiting room somewhere. It doesn't matter if it's a doctor's office or the DMV or some other place. But it's a boring office-ish building, and you're just sitting there.  As you struggle to keep your eyelids open, you hear a fire alarm go off. It's unmistakable; you've heard this exact noise before. This is definitely a fire alarm.

But...I do not know for sure how you would react next.

Maybe you would just head straight for the exit. Maybe you would make sure all the children had an adult helping them get to the exit. Maybe you look for puppies to rescue. Maybe you try to find the fire and fight it, personally, with your fists.  Who knows. 

But...maybe you think the fire alarm is overly-sensitive and detecting the break room microwave's popcorn, or that it's just a fire drill. Maybe nobody else in the waiting room is running out the door, except that one very skittish family, so you figure you shouldn't leave, either. So you keep your seat, or cover your ears. Maybe you look for the alarm to turn it off. I don't know.

But the point is: whatever you believe in, you will always act consistently with that belief. Even if you are consistently being a liar or hypocrite. Someone who doesn't think there is a fire might still act like there is a fire if they think they can get social media points by filming their own panic, because they are consistently valuing those virtual "thumbs up" icons more than living consistently with the truth. Or someone who thinks there is a fire might be afraid of whoever they are with, and don't want to be yelled at / punched by them --- they are living consistently by fearing their companion more than they fear the fire.