Why are there so many divisións in Christianity? Why can’t we all be one?
I have already answered this question, and am copying an earlier answer but am amplifying the response a bit.
The reason there are so many denominations is a matter more of history than of anything else. There really was just one Christian church in the first three and a half centuries. From about 400 AD until about 1700 AD, Christianity was governed by state-run churches. Early on the single Catholic Church divided into Roman Catholic, Greek Orthodox, Coptic (Egypt), Nestorian and Jacobite (Church of the East). This division was over relatively minor disputes over theology, but language was also an issue. Catholics used Latin, Greeks used Greek, Copts used Egyptian/Coptic, and the Nestorians and Jacobites used Syriac. During that period the number of Christian groups was limited and competing Christian groups were generally suppressed. The Reformation brought in a wave of new groups, but not all that many because in this period there were principally only state-run churches. As of 1700 AD there were probably a bit more than a dozen definable Christian groups (Coptic, Roman Catholic, Jacobite, Orthodox, Lutheran, Reformed/Calvinist, Anabaptist, Quaker, Puritan, Brethren, Baptist and a couple more).
After 1700, as religious tolerance was gained in Holland, America, England, and later throughout Europe and much of the rest of the world, a flood gate was opened and literally hundreds of separate definable Christian groups sprang up. We can argue that this is a very bad thing, as Jesus pleaded for Christian unity (especially in John 17). However, as long as human beings are free to act, new Christian groups will continue to pop up. I believe that no effort to stop the formation of new definable Christian groups will work at this point, as there is no body of humans able to force groups to come together under one organization. It is even possible that some of the diversity is not all bad, as different needs of different personalities and cultures may be met by a diversity of groups. I am not saying that division is good but some diversity is not necessarily all bad.
Many teach false doctrines. Some of the variety is because of disagreement over essential doctrines. The existence of false doctrine is part of the reason for the division into so many Christian groups, but many differences are over inessential teachings. The existence of false teaching is one of the explanations of all these groups. However, some of the division is over inessential teachings. It is hard for a charismatic believer to be in intimate fellowship with a believer who does not accept the validity of present-day speaking in tongues and other miraculous manifestations. It is not likely that charismatics will convince all believers to allow the use of these gifts in their churches because many do not agree that they should be practiced at all. There are many other unessential teachings and practices which, for practical reasons, will probably continue to prevent the coalescence of disparate Christian groups. So, as lamentable as the amount of division in Christianity is, it is not likely that these divisions will end any time soon.
What a sincere Christian can do is to find the group most in line with what they believe proper Christian teaching and practice is, but also a group which is making a difference in this world and join this group. Once we join a group, we should be a force for unity, not division. We should look for ways to bring different groups together as much as we can, but we should also understand that it is not reasonable to expect all Christian groups to simply drop their walls and for a single worldwide fellowship. Lamenting the current situation is not all that helpful, but working for as much unity as possible is what Jesus would want us to do.