Divisions arise for two reasons: (1) saved vs unsaved and (2) true doctrines vs false doctrines. And as both Jesus and Paul said, divisions are unavoidable. The Catholic church went off the rails a long time ago. And later on the Protestant mainline denominations also went off the rails. So every person must study the Scriptures and see what the various denominations believe and practice.
Several churches in the first century went off the rails - it was no different than today!
Keep in mind the Seven letters to the First Century Churches. Two were righteous and Jesus did not rebuke them, Philadelphia and Smyrna.
Ephesus was an orthodox church with good doctrine, but forgot their first love. This was to be expressed as brotherly love. Pergamos was a worldly church with false doctrines, sin and materialism.
Thyatira tolerated sin and was undisciplined.
Sardis was a dead church with no fruit or growth and only a few in that church were saved.
Laodicea was lukewarm, self-sufficient, in their comfort zones, tooting their own horns when they do something nice, went to church to be seen and accepted and gave out of their abundance. Many Christians today live like the Laodiceans.
These types of churches did not just exist back then, these were all Church-Types that existed throughout the Church Age.
Most of us would prefer to identify with the Philadelphia church. I feel my church was like this, a faithful church.
In some Islamic countries, Christians are severely persecuted - like in Smyrna.
Also when we try to consider how divided we are, why we separate ourselves, we can further dissect each church's individuals. Each one of us within each church holds different beliefs, has their own sin and path that is unique to the them. Within one church you can have a group who are faithful like the Philadelphians, but also a few like any of the rest. We all have faults in every church no matter what denomination. Yet Christ is forgiving and factors all our blunders into His plan. As long as we have faith in Christ, the Author of our faith will finish it. Spiritually, as born again Christians, the Body of Christ is pure. It is our flesh where sin dwells and we will soon be rid of that.