You know how we have words that everyone knows what they mean and, yet, no one is quite sure? No? Well, I would classify "Christian" as one of those words. Sure, sure, you can see that word anywhere. It is applied to churches, to political groups, to individuals. It is so loosely applied that it seems to have lost its meaning.
Barna lists five "faith segments". Three of them are listed as "Christian". There are the Evangelicals, the Born Agains, and the Notionals. Evangelicals are "a group of individuals who believe that their relationship with Jesus Christ will provide them with eternal life, and who accept a variety of Bible teachings as accurate and authoritative." This is the smallest of the three that comprise "Christian". The "Born Again" group "believes they have eternal salvation through the grace given them by God through their personal faith in Christ, but do not believe in various core doctrines taught in the Bible." The third is the "Notionals" "who consider themselves to be Christian, but do not claim they know their eternal destiny (i.e., whether they will experience eternal life, eternal damnation or some other outcome) and are less likely than others to embrace core Bible doctrines." In the world of American "faith segments", Notionals make up 44% of the nation, Born Agains another 33%, and Evangelicals are at 8%. The rest are not classified as "Christian".
Evangelicals are quite different from all the other groups. They derive their unusual moral beliefs from the Bible and are more active in religious activities such as Bible reading, church attendance, praying, and sharing their faith with non-believers. The Born Agains, on the other hand, get their moral guidance from other sources including, most of all, personal feelings. They go to church and all that, but at a much lower rate than the Evangelicals. And they aren't nearly as likely to share their faith with others. The Notionals are much more "normal" -- like the world. They think they're likely "saved", but "not because of a grace-based relationship with Jesus Christ." Only one in ten gets their moral beliefs from the Bible. Not quite 40% consider themselves "pro-life". They attended church as a child but no longer. They generally don't believe the Bible is accurate, don't believe in Satan, and don't believe in the Holy Spirit.
"Christian", on the other hand, has an actual meaning. It refers to "a follower of Christ". It is one who is identified with Christ. And Jesus was abundantly clear. "Truly, truly, I say to you, unless one is born of water and the Spirit he cannot enter into the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit." (John 3:5-6) He said, "Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God." (John 3:3) That is, key -- absolutely critical, indispensable -- to being a "Christian" is being born of the Spirit.
Interestingly, what our Bibles translate here as "born again" are two words: γεννάω -- gennaō -- and ἄνωθεν -- anōthen. The first means "born", but the second is not literally "again". It is "from above". (In fact, many Bibles have that in the notations.) While Peter speaks of God "who according to His great mercy has caused us to be born again to a living hope through the resurrection of Jesus Christ from the dead" (1 Peter 1:3) and uses a term that means literally "born again", this is not the term that Jesus used. This new birth is "from above". As such, this new birth cannot fail to change those who experience it. Jesus said, "If you love Me, you will keep My commandments." (John 14:15) Notice that He did not make it an imperative. "If you love Me, keep My commandments." No, He put it in the indicative. "You will keep My commandments." It isn't a question. It is an indication. John said, "No one who is born of God makes a practice of sin, because His seed abides in him; and he cannot sin, because he is born of God." (1 John 3:9) John says that one who is born from above lacks the capacity to be comfortable in sin. Jesus defined eternal life as a relationship with God (John 17:3).
We've shifted words over time, as I've often pointed out. "Christian" is not immune. Satan would like to shift it into meaninglessness like so many other terms. He has been largely successful. But it still has an original meaning coined in Antioch of those who were marked as followers of Christ. And we ought to be using the term the same way. Lots of people who classify themselves as "Christian" fail to meet the criteria that Jesus placed on the term. Many don't even know it anymore because the term means so little. But if you wish to classify yourself as "Christian" and have it have any significance, you will need to do so by being a follower of Christ. "Notionals" are not. Those who call themselves "Christian" while ignoring God's Word are not. And believing yourself to be a "Christian" while not is a genuinely dangerous place to be.