Notwithstanding centuries old divisions between Christian churches, to say nothing of millenium old divisions between churches (filioque clause, monophysitism ...), Christian unity in the gospel is possible because Christ prayed for unity among his followers. It is inconceivable that Christ would pray for something not possible to occur. Further, the mature Pauline theological vision of the great purpose of God for the world is summed up in one word: unity.
'according to his purpose, which he set forth in Christ, as a plant for the fullness of time, to unite all things in him, things in heaven and things on earth.' (Ephesians 1:9-10)
On the one hand, Christians are divided unnecessarily by conceiving the gospel too narrowly. When the New Testament itself offers at least five versions of the one gospel (Matthew, Mark, Luke, John, Paul), our talk of what the gospel is in respect of its content and meaning needs to allow for Holy Spirit-led 'diversity-in-unity.'
On the other hand, Christians are divided unnecessarily by insufficient patience in theological exploration. Many apparent divisions among us represent differences in presuppositions, or understanding of history which, with more care about the starting point for our discussions could lead (should lead) to converging unity in doctrine. To an extent, recent discussions in the past half century or so between Anglicans and Roman Catholics, between Roman Catholics and Lutherans, and between Roman Catholics and Eastern Orthodox, along with some World Council of Churches work on 'faith and order' have demonstrated the possibility that with time and a fair wind of the Spirit, we could be one undivided church again. But it will not be in my lifetime!
I want to suggest here that we are also divided because we keep taking our eyes off Jesus Christ and lack focus on Christ as the centre of the gospel. It is a simple fact, for instance, that the church is the body of Christ. All of its ministry and mission flows from Christ. Any division among us over ministry order, sacrament, and the like, is misunderstanding about Christ in the life of the church: it is Christ's episcopacy, for example, and his sacrament. Not ours.
There is diversity in the body of Christ (as Paul teaches, 1 Corinthians 12), but no division, for Christ is not divided. Where there is division in the visible church then something is wrong. Responsible Christianity works to fix the wrong, not to tolerate it, nor to attempt to paper over the division.
(Incidentally, Preludium offers a very interesting post on a divisive issue of the day, women in episcopacy in the C of E. But both Mark Harris, and the men he takes to task, may be missing the point! The question is not whether democracy will prevail or should not prevail, but whether starting with Christ and remaining centred on Christ we discern in unity that his oversight of the church ('the great shepherd of the sheep') may be expressed on earth by any suitable person redeemed by Christ into his body). (Later: with a very good reply by Mark Harris to my point)!