What is a Christian?
Where I come from, just outside of New York City, there is a popular caricature of what faith looks like. There, you have many people who are "religious." In other places and times, a person who was particularly devout in prayer, consistent in church attendance, or at the very least, very moralistic would have been considered "religious." The moniker "religious" has come to mean, in the context of the local parlance, that a "religious" person is simply someone who believes in the existence of God. A "religious" person may not (indeed, probably does not) attend church, is not devout by any historical standard, and their morals are likely shaky.
The contrast to the "religious" person is the "born-again" person. The differentiation is quite apparent if you ever have the occasion to visit the greater tri-state area; you will likely hear someone refer to a devout Christian as "OneADemBornAgains" with the same brogue and vitriol that one might say 'Fuggedaboutit." These folks are known for their church attendance, sincerity of faith, and some kind of lifestyle that stands out as comparatively devout. "DemBornAgains" (another linguistic permutation) are likely to be Protestant Christians in most cases.
With both of these groups shrinking dramatically over the past decades, the question persists: "What is a Christian?" Is it enough to have a general faith in God to save someone from judgment? Do I truly need a faith that is so robust that all my doubts are gone? Do I need to start listening to Christian music and watching Fox News in order to be "saved"?
What a Christian is Not
It is widely assumed that "religious" people who are morally restrained, that is, they "don't drink or chew or go with girls who do," must be God's elect whom He is going to save from destruction. Jesus consistently declares that there is nothing you can do to save yourself and that only God can save you (John 3:3, John 6:44). Moralists see their behavior and their uprightness as something that deserves God’s love and mercy rather than the recipients of the free grace of God.
Does a general belief in God count as saving faith? No, it doesn't. Jesus said, "the demons believe there is one God, and they tremble" (James 2:19). A cognitive decision that there is a God cannot save you. The demons believed and were trembling! Just admitting that God exists does not constitute a saving faith any more than your belief that bears exist would save you if you got in a fight with one.
Ah! You say a Christian is someone who thinks they may make some mistakes, but they possess, robust, intractable, bulletproof faith! This is also not so. Jesus consistently taught that just a little faith was enough to move mountains. There are droves of people who flirt with the Christian claims, but they essentially give up when their lives, psychology, worldview are not instantly changed. When faith seems like a struggle, they jettison the whole idea out of a sense of failure or dejection. This caricature is no more true than the others. Christianity teaches that it is the object of your faith that saves you, not the quality of your faith. Militaries around the world use something called parachord; it is unbelievably thin and equally strong. A 250lb man would have no problem rappelling on a single strand, which is less than ⅛ an inch thick. I remember when I was a junior Marine and this was being explained. I was skeptical. I didn't have a whole lot of faith that this thin cord would support me. Another way of saying it is that the quality of my faith was poor. Thankfully, the object of my faith (the parachord) was unbelievably strong, and so the object of my faith saved me (from falling 4 feet), not the quality of my faith.
If the object of your faith is Jesus, then you are in good standing. If you are relying on Him to save you through His matchless life and His death on the Cross, then the object of your faith is something certain and imperishable. Even if the quality of your faith ebbs and flows as you wrestle with doubts and experience life, the object of your faith, Jesus, never wavers in His resolve to pay the price for anyone who calls on His name.
“...a Christian is someone who has seen, at the same time, the depths of their moral unworthiness and the love of Jesus which covers the sins of anyone who sincerely calls on Him to do so.”
What Is A Christian?
The late Dr. Timothy Keller summed it up brilliantly when he explained that "Christianity is not the good are in and the bad are out; it is the proud are out and the humble are in." A proud person will be hard-pressed to admit their faults and failures as it works against their governing ideal. If you believe that you are exemplary, noble, and just, you will unconsciously suppress any evidence to the contrary. This carries the consequence of making us insufferable towards other people whom we deem as not as deserving. This creates a kind of feedback loop where we continue to delude ourselves into thinking we somehow deserve the gift of God's grace.
Fundamentally, a Christian is someone who has seen, at the same time, the depths of their moral unworthiness and the love of Jesus which covers the sins of anyone who sincerely calls on Him to do so. A Christian must necessarily have at least enough humility at the outset to admit that they need help. A Christian is someone who, like the thief on the cross, cries out in his darkest hour, "remember me when you come into your Kingdom." There is no need for eloquence, just sincerity.
Jesus compares faith to a seed planted in the ground. Just like a seed, a sincere faith will germinate and grow. As it does, you may see growth in obedience to the way the Bible calls Jesus' followers to live, you may sense an increasing enthusiasm to be with a covenant community of Christians who at one point probably seemed a bit weird to you. This is the work of the Holy Spirit who inhabits the psychology of anyone who is relying on Jesus. It doesn't happen overnight, and it's not completed this side of heaven, but, as Martin Luther put it, "we are saved by faith alone but not by a faith that remains alone."