Jesus prayed for the church he bled and died for, that she would be one (John 17). He said several chapters earlier, "so there will be one flock, one shepherd" (John 10:16). Likewise, the apostle Paul begged that the church in Corinth would agree with one another that "there would be no divisions" or "quarreling" among them (1 Corinthians 1:10-11).
Since the earliest days of the church's existence, Christians everywhere have confessed one holy, catholic, and apostolic church. So what on earth happened to this grand, but foiled vision for unity that all of the church's founders and earliest followers had? Why is Christ's church so divided?
It's important to state upfront that Christian unity is not defined by anything other than the gospel. The very gospel that establishes the church is the basis for unity among Christians. As Paul seems to emphasize to the churches he planted in Galatia:
I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel—not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed. (Galatians 1:6-9)
Christian unity is founded upon the gospel—namely, that Jesus Christ "gave himself for our sins to deliver us from this present evil age" (Gal. 1:4). As Paul clearly teaches above, even a human institution established by the apostle himself is able to depart from such a gospel. If the apostle himself shows up declaring a new message, he's anathema ("accursed").
Besides, if there's anything we learn from reading the Bible—it's that Christians get divided easily. Paul and Mark parted ways. Peter and Paul had a few problems with one another along the way. The Galatian churches were departing from their own constitution. It seems likely then that, as time goes on, more division and schism is going to continue.
We don't only read of this in the Bible, but we experience division daily. There are over 217 separate denominations in the United States today, and over 35,000 independent or nondenominational churches that, for the most part, have no connection or communion with one another. Drop a theology bomb on Facebook or Twitter and watch all the Christians pick up and argue over the pieces, the reasons why, their differences. Division and schism is a common experience within and without the church of today and yesterday.
Most church going Christians have attended multiple churches over the course of a lifetime. Sometimes we leave a church over bad theology, but usually something bad happens. Sexual abuse, rape, emotional abuse, or we don't like someone, or we are not liked by the "in crowd." The church is messy.
The church itself is created by the gospel, upheld by the Spirit of Christ (i.e. the Holy Spirit), and led by ministers, elders, and deacons all over the world in every language, ethnicity, and nation. Institutions exist all over the place claiming the name of Christ and the truth of his gospel—but many false teachers and churches abound within the ranks of these institutions.
Again, Christian unity is established and maintained in, by, and through the gospel alone. There is no single institution on this earth that is immune to losing this gospel. Any church can have its lamp stand removed. Just read the first several chapters of Revelation, Jesus says so.
So why is the Christ's church so divided? I suppose I haven't really given an answer to that yet. The greatest reason for such schism in the church is that three letter word, "S - I - N." New churches, denominations, pre-denominations, post-denominations, etc. spring forth because of personality cults and clashes, politics, pride, and power. It's a combination of all of these things (and more), but it's certainly no less than that. Human sin is the reason, the source, and the result of division in the body of Christ.
Why would God let this happen? How could the Son of God allow for this legion of demons to dwell within his beloved Bride and wreak such havoc on her? How can we trust such a Lord? Well, the most likely explanation for all the division we see in the church comes down to this: that no one may boast save in the death and resurrection of the Lord. That is, after all, the solution to division that the apostle Paul gave to the divided church in Corinth (1 Corinthians 1:31).
My church did not save me. My denomination, the Presbyterian Church in America (PCA), will never save me. My former denomination, the United Reformed Churches in North America (URCNA) could not save me. My favorite celebrity pastor definitely won't save me. Heck, even the oldest surviving church building in Syria won't save me. Jesus Christ alone has saved and will forever keep me. No human institution will ever be more worthy or powerful than the one who was crucified for me.
It's impossible for Christians to boast in our greatest sense of unity. The most unified international, or national, or regional, or local Christian bodies on this planet still experience plenty of division. The most unified Christian church in the world has fractions and factions within her ranks—it's not hard to find.
And all of this goes to further show that although Christ's church is imperfect and fractured, Christ himself is perfect and healed—He is after all called, The Great Physician.
Heal us, and Lord, have mercy on us.