Look up "apostle's creed" with a search engine and you'll find a number of sites, many of which have a special footnote at this point clearly pointing out that "catholic" does not mean "Roman Catholic". It means "universal". The Body of Christ is the Church, with a capital "C", made up of all believers, or "saints", everywhere. One common knock against Christianity is the amount of division that exists within the supposedly unified Church. Critics point to long-standing hostility between some members of the Roman Catholic, Orthodox, and Protestant churches. They point to the multitude of Protestant denominations, a number of which claim to be the only true Church, and that anyone outside their particular denomination is lost. There are a fair number of Christians on either side of practically every spiritual and social issue. The Bible claims that God is a God of order, not confusion. Why all the conflict?
The answer is simple, though not very pleasant to state or hear. While many people have an idealistic belief in the basic goodness and perfectibility of humankind, the hard evidence of history supports a much more dismal view. In general, people are selfish, lazy, arrogant, insecure, inconsistent, illogical, and ignorant. We prefer to do that which provides the most benefit to ourselves with the least effort. We conclude that anything we can't easily understand is probably not true or not worth thinking about, and we will often let someone else do our thinking for us. We react to situations emotionally rather than taking considered action. Once a position has been taken, our pride convinces us that position is "the best" and "the only". No parent has ever had to teach their children to think of themselves first. No utopian community has ever succeeded for long. We honor people who act unselfishly, who work hard, who are committed despite adversity, who are humble despite achievement, precisely because we know how rare those qualities truly are. The Bible is unstinting in its description of humanity in terms such as these. The technical term for all this is "depravity". We aren't very good at knowing our own selves. How can we be expected to have a complete and accurate understanding of God?
As if the personal failings of Christians themselves weren't bad enough, God has other enemies. Chief among those is Satan. He is doing his absolute best to draw people away from the truth, and it often isn't very hard. There are plenty of people out there who hate the Church so much, who are so opposed to the very idea of God, that they will go to any lengths to tear the Church down. There are others who really don't care so much about the Church itself one way or another, but are perfectly willing to take advantage its existence. They seek power, money, or fame and will take it from believers in the guise of preachers or prophets or from non-believers in the guise of "historical scholars" purveying evidence of Bible falsity. All of these enemies of truth prey on our basest instincts and our noblest ideals. We are too often led astray by what we want to believe is true, rather than what has been shown to be true.
It is clear that whatever unity Christians have exists only because of the Bible. Whatever meanderings our tiny little souls take us on, all who call themselves Christian must start there. People might depravely add to it, subtract from it, twist it, and torture it, but without the Gospel there is no Christ. Without the authentication and foundation of the Old Testament, the New Testament has no meaning. The clearest statements the Bible makes about what it means to be "saved" indicate that the true Christian must believe the following things:
If you believe all these, and have actually gone through the acceptance phase, it would be extremely difficult for anyone, based on purely Biblical grounds, to say you are not a Christian. If you don't believe any one of them, it becomes very easy to say you are not a Christian. If you don't believe either of the first two, then you cannot believe that the third is necessary or meaningful. If you don't believe the third, then you must also believe that Christianity, as a belief system, is completely unnecessary, which is a repudiation of Christ's sacrifice.
Now, I understand that the three criteria are "loaded". There are a lot of things one must believe before one can say they believe each point. Each of the terms like "God", "sin", "holy", "Savior", and "Lord" must be well understood, especially "Lord". Practically everything that divides Christians is derived from that statement of authority. More precisely, Christians are divided over how authority is derived from the Lord. The Roman Catholics claim that supreme spiritual authority among men has flowed in an unbroken line from Christ, to Peter, to John Paul II. The Orthodox Churches have their Patriarchs. The Protestant churches vary in their systems from hierarchical to congregational to individual. Some claim continuing revelation, some claim special revelation. But, all conform to the idea that one's behavior is the true indicator of one's beliefs, and that to go against "authorized behavior" calls those beliefs into question.
But, how can we test the quality of the authority? Obviously, not everyone who claims to speak for God can actually be doing so, since they often contradict each other. What criteria can we use? Again, we are left with the one source of information which doesn't change and to which every Christian organization must give at least some credence. Any pronouncement, principle, or practice which is not clearly supported by an informed reading of commonly accepted Scripture is suspect and should not be used to judge a person's ultimate fate. We must also come to the realization that many of the things that we allow to divide us are relatively unimportant. If my salvation is threatened by using grape juice instead of wine during communion, or including rock music in worship, or saying the wrong prayer at the wrong time, then that salvation is a weak and fragile thing in which I can never be secure. Christ is not so small, nor the Holy Spirit so delicate, nor our Father's heavenly grace so ineffective as to be washed away by anything other than my utter and willful rejection. I do not always please my Lord, but he remains my Lord nonetheless and I remain one of his saints, as do all who call upon His name.