If you were to open up the yellow pages looking for a church, it would be easy to get confused with all the different churches that are out there. So to answer this question, the first thing we need to do is make a distinction between religions, denominations and cults. We can define religions as a way for human beings to relate to God, or whatever they consider sacred or supernatural. Islam, Hinduism and Buddhism are examples of entirely separate religions from Christianity.
A denomination, put very simply, is a specific religious group. For the context of our discussion let's refer to denominations as denominations within the body of Christ such as Presbyterians, Baptists and Lutherans.
The word cult can be widely misunderstood. When I use the word cult here, I am referring to a non-Christian cult. Again for our purposes, I would define a cult as a group claiming to be Christian but denying one or more of the essentials of the Christian faith. Mormons and Jehovah's Witnesses are examples of cults. They are cults because they deny essential biblical teachings and add new, false doctrines.
So, why are there so many different denominations? My Roman Catholic friends love to make this charge against us Protestants. They say, "Look at what your doctrine of sola scriptura has done." What is sola scriptura? The phrase sola scriptura is from the Latin: sola having the idea of "alone," and the word scriptura meaning "writings" - referring to the Scriptures. Sola scriptura means that Scripture alone is authoritative for the faith and practice of the Christian. The Bible is complete, authoritative, and true.
I'll admit, the number of Christian denominations present an issue for those of us who hold to the doctrine of sola scriptura. Let's take a look at 1 Corinthians Chapter 1. "I appeal to you, brothers, by the name of our Lord Jesus Christ, that all of you agree, and that there be no divisions among you, but that you be united in the same mind and the same judgment. For it has been reported to me by Chloe's people that there is quarreling among you, my brothers. What I mean is that each one of you says, "I follow Paul," or "I follow Apollos," or "I follow Cephas," or "I follow Christ." 1 Cor. 1:10-12. For us today it would be like us saying, "I follow Luther," or "I follow Wesley," or "I follow Calvin." According to the apostle Paul, there is only one church and one body of believers. We shouldn't be divided, or even worse, quarrel and argue amongst each other. When we do, we act carnally. For when one says, "I follow Paul," and another, "I follow Apollos," are you not being merely human (carnal)?" 1 Cor. 3:4 Jesus desires unity in His body, not division. (John 17)
To answer this charge we have to go back to the Protestant Reformation. A man named Martin Luther was at the center of the controversy. He was the father of the Reformation, and he publicly rebuked the Roman Catholic Church for its unbiblical teachings. They threatened Martin Luther with excommunication (and death) if he did not recant. Martin Luther's reply was, "Unless therefore I am convinced by the testimony of Scripture, or by the clearest reasoning, unless I am persuaded by means of the passages I have quoted, and unless they thus render my conscience bound by the Word of God, I cannot and will not retract, for it is unsafe for a Christian to speak against his conscience. Here I stand, I can do no other; may God help me! Amen!"
Martin Luther was prepared to die so that the Holy Scriptures could get into the hands of everyday people. He believed so much in the Scriptures, that he wanted everyone to have the right to read them for themselves. He was willing to make a trade-off: let everyone read the Bible for themselves, instead of only the clergy having access to it, but this however, will inevitably lead to different understandings and interpretations. I believe he was willing to make that trade. It is of the utmost importance to give the Bible to the people.
Denominations stem from disagreements over the interpretation of Scripture. My belief is that we can disagree, but we can't argue, or quarrel. Let's discuss those areas where we have differing interpretations, but let's recognize that Christians are one body, and we should love one another, and I believe true followers of Jesus understand this. As believers, there are certain basic doctrines that we must believe, but beyond that there is latitude on how we can serve and worship. This can be classified as diversity not disunity.
Personally, I belong to a non-denominational church, and although I am reformed in my Theology, I do not want to be referred to as a Calvinist. I follow Jesus Christ not John Calvin. The fact is those who do follow Jesus Christ and who pursue truth and desire to know and understand God's Word, those people, generally do agree.
The following is a list of the doctrines that Christians have come to believe in as a whole, and we don't have any disunity when it comes to important doctrines such as these:
God - God is Triune in nature, yet He is one. He is Father, Son and Holy Spirit; all of whom are equal, yet with some distinct roles.
Jesus - He is eternally God and nothing less. He was never created, and is equal to the Father and the Spirit.
Holy Spirit - He is not a mystical force, but a person of the Godhead equal to the Father and the Son.
The Bible - The Bible is the complete, perfect, infallible, inspired Word of God. There are no missing books and no mistakes.
Salvation - We can only be saved by repenting of our sin and placing our trust in Jesus as our Savior.
Christians have agreement on ALL of these.