In New Testament days, denominations did not exist. Paul could write to the Christians in Corinth or the Christians in Philippi or the Christians in Ephesus. Denominations have sprouted over the history of the church for a number of reasons. Some have formed over differences in doctrine, some over differences of structure or practice. However it has happened, our situation today seems very far removed from the kind of unity that the New Testament talks about and which God so much desires.
Most denominations have little to do with each other.
Artificial barriers, caused by denominationalism, abound in every local community. It is the form that religion takes in a culture controlled by the ideology of the Enlightenment. It is the social form in which the privatisation of religion is expressed. These are scattered, fragile, and vulnerable enterprises, but they indicate the direction in which the church must go. Obviously, God loves diversity, and diversity should be no problem in the church.