If you put all the thousands of different, and in some cases incompatible, doctrines of Christianity in a crucible so that every single disagreement among the various sects and denomination is vaporized, what you're left with is a handful of basic core beliefs that all Christians, whether fundamentalist or liberal, believe in. They believe that Jesus is the Christ (Messiah, Son of God); they believe that accepting Jesus as your Savior is your ticket to heaven, and of course, they believe that God actually exists as an all-powerful, omniscient and personal being. But in writing these things, it occurs to me that some of the fringe Christians even have trouble with some of these. Universalists believe that all will go to heaven, at least eventually. Some ancient sects of Christians believed that Jesus was just a great teacher and was not divine. But for the sake of argument, let's assume that at least the vast majority of modern Christians believe at least these three things.
All Christians also believe, to some degree, in the the authority of the bible. Some believe that it is literally true and universal; others believe that it is mostly allegorical, but the reason they all share their core beliefs is because those beliefs are prescribed in the bible.
As a nonbeliever I take issue with all these core beliefs that Christians share, but I'm most concerned about the second one in the list above. Christians say that you must believe that Jesus is the Son of God and accept him as your personal Lord and Savior. It sounds simple enough. All the other commands that are found in the New Testament are descriptive (such as in Acts), or prescriptive to a particular church that has gone astray in some way, such as in Paul's epistles. But the act that makes you a Christian is not how you behave but what you believe. The theory is that if you truly believe in Christ then the Holy Spirit will enter you and guide your behavior as well as provide you comfort. But first comes the believing - faith.
There is one major flaw in this logic: What if someone can't believe. For the most part, what you believe is not so much chosen by you as it is thrust upon you. Mostly it depends on the culture you were raised in. If you were born and raised in the American South, odds are very high that you will be an observant Christian and will have no problem at all believing in Jesus as the Son of God. If you were born and raised in New England to parents who are professors at Harvard, odds are that you will have a tougher time of believing anything the bible says about Jesus' divinity. If you were born and raised in Saudi Arabia you would most likely believe that Jesus was just a minor profit. And regardless of where you were born, if you can step back and look at all the religions of the world from a neutral position, you might decide that all of them are bogus, relics of a superstitious era that no longer exists. In this case you would view the bible as a quaint, yet potentially very dangerous, holdover of an earlier age.
In essence, then, what you believe is shaped over time by your environment and is not a matter of instantaneous choice. How can you accept Jesus as your Lord and Savior if you already believe that there is no personal god, or that Mohammad is the savior of mankind, or that Vishnu is one of the gods you need to follow? That's not to say that people can't change their minds, but in most cases, a mind change of that magnitude normally requires that a good reason be provided. But different people are compelled to act based on differing degrees of evidence. Those who lack critical thinking skills can be swayed much more easily than those who take the time to think things through more thoroughly.
If a friend told you a story of how he was abducted by space aliens and given a code of ethics that, if followed, would guarantee an eternal life of bliss would you believe him without an enormous amount of evidence? What if this friend wanted to persuade you to follow his alien code of ethics, which included a rejection of all earthly religions and an oath to affirm a belief in the alien society, would you choose to follow his advice? A rational, thoughtful person would tend to believe the friend may have been high on drugs or was having some sort of psychotic breakdown. A dullard from Podunk, Arkansas might be tempted to join up, except that he, like most other Podunkians, already have a delusion to follow - Christianity. The upshot is, most people, no matter what their current religious affiliation, would reject the friend's effort to convert them. That's because most people can't simply choose what to believe without compelling evidence. And some people's belief systems are so ingrained that even evidence to the contrary is useless.
I understand the concept of Christianity; I was born into a Christian family and raised as such. I have been baptized, as an adult, and lived most of my adult life with an open mind about Christian beliefs. But in looking back, most of what I admitted to believing was simply lip service. I wanted to believe in an eternal life in heaven. It still is very appealing to me. I would like to have the joy and peace of mind that most claim come with believing is Jesus Christ as Savior. But, alas, I know too much. I know that there are too many people in the world who belong to a host of other religions that make the same claims about their beliefs as Christians claim about Christianity. I can step outside the bubble of the religion I was raised in and see that all religions are basically equal. None of them have evidence to back up their claims. All of them expect a belief in a supernatural entity. And all ascribe things to this entity that can more readily be explained by science, using only nature. No matter how much I would like to believe in a benevolent god who offers everlasting life, I can't actually believe it because I know better. There are hundreds of really, good, solid reasons why I can't believe in the Christian doctrine. The only reason I have FOR believing is a promise of an eternal life of joy. But it's a hollow promise with no evidence to back it up.
So when Christians come up to me and tell me that they will pray for me so that my eyes will be opened to the truth, I tell them it was the truth that turned me away from a belief in superstition in the first place. It is not up to me to choose to believe. If it were, I would choose that option. But in the same way that you can't simply choose to believe in Santa Claus (because you know he's not real) I can't choose to believe in the Christian god, or any god for that matter. And it is unreasonable for A Christian to assume that a nonbeliever, or someone who has a different religious delusion, can simply choose to believe in their god. And if there were a god, he would already know how unfruitful it would be to ask someone to just believe.