To many people, the presence of so many denominations of Christianity is evidence of extreme division within the body of Christ. However, this is only part of the story. The existence of denominations, oddly enough, is actually a help to maintaining unity in the Church. Before I explain why, I would like to point out some groups that are not denominations, but are cults or similar, heretical groups. Some of the most prominent include Mormons, Jehovah's Witnesses, and Seventh Day Adventists. These are not part of the Church and thus irrelevant to a discussion on denominations.
Anyway, my point is simple: denominations are necessary for unity among believers. They are not terrible dividers. The reason I say this lies in the nature of denominations. For the most part, every denomination is distinct from other denominations in a way that would make members with the various beliefs assembling together in one church annoying, difficult, or even entirely unworkable. As an example, consider speaking in tongues. Pentecostal denominations such as Church of God consider speaking in tongues to be an important part of church vitality. Other denominations such as most Southern Baptists believe that tongues is no longer meant to be part of the church at large, and thus most of the activities in Pentecostal churches either originates in an emotional, ecstatic experience created in the mind or (as some believe) even demonic sometimes. Obviously, if there were no denominations and all believers assembled in common churches, this would bring strife. If half the "Church of Generic Town" believed as Pentecostals do and half believed as most Baptists do, the church would be divided and filled with strife. However, as things are with Baptists and Pentecostals meeting separately, the believers of differing opinions are free to coexist in the same region in harmony, able to interact amiably between the two churches without personal conflict within either church. The same applies to baptism. Most denominations of the Reformed tradition believe in infant baptism, while others, especially Baptists, only believe in believer's baptism. A church consisting of different groups with both of these beliefs would be prone to serious problems arising when part of the church denounced another part for baptizing their babies and that part denounced the others for not baptizing them. Baptism is such a serious part of the church life that this kind of controversy could not be peacefully sustained in one congregation. By allowing Presbyterians and Baptists to have separate assemblies in one place, the two groups are free to practice as they believe the Bible teaches, and they can interact and cooperate as they wish without creating internal division in either church. I could go on about numerous such controversies regarding music, theology, and other elements of the Church life. In the end, if we did not assemble by denomination, we would have far more internal divisions in individual churches. As it stands, the distinctions between denominational beliefs are often held less personally than they would be if all attended the same church. This allows for greater freedom of cooperation and tolerance of various church practices and beliefs within orthodoxy. In conclusion, while most think that denominations divide, in reality denominations are vital to stable unity in the body of Christ.