John 17:20 - 23 describes a totally unified Chruch - no divisions - we are to be perfectly unified like Christ and God are unified - why is this so hard? Why are we the way we are now as a body?
Unfortunately, human beings are fallen beings that are imperfect due to sin, and we have a limited understanding of spiritual truths - as such different people interpret bibles in different ways, leading to different denominations, sects, even cults.
However, I believe as we near the end days, the church will become more and more like the church in Acts.