Unity is a challenging issue for Christian churches, since they haven't been able to agree on one single thing since their founding 2000 years ago. Indeed, it wasn't that long in the space of history, from its inception, that the original church split into two factions, Catholic and Orthodox and from there, off it went.
It could all have been prevented if "called ministers of God" would honor God and His Word as true and the ultimate authority, instead of trying to shape it all in their image to please the itching ears of their congregations....
Squabbles over doctrine, methods of baptism, which day is the Sabbath Day, the gifts of The Holy Spirit, the elements of Communion (is it symbolic or do the elements really become the blood and body of Christ?), sacred music vs.slam-bang modern choruses, Calvinism, The Trinity...length of hair and makeup has all resulted in heated debate over the centuries as new denominations were constantly appearing on the scene with their own particular take on The Holy Scriptures.
Churches base their doctrine on specific Scriptures, like the 7th Day Adventists whose main doctrine is when to celebrate the Sabbath....the strange snake handling cults that base their message on, "...and they shall pick up serpents (without harm), " Mark 16:18, and certain holiness churches that believe anything that is not worshipping God is sin...the list goes on and on.