Jay is right to point out that Christians are disunified. We have denominations that have traditionally fought over the most minute, and the most serious, of theological positions. One believes that child baptism by sprinkling is enough, while another insists on an adult immersion. One gives communion weekly (or more), another quarterly, and a few only yearly. Some say that a congregation's leader must be married, some prefer it, and some forbid it.
Being at peace with each other is the most important kind of unity Christians can have. Paul and Barnabus got into an argument over taking Mark. Paul was right in that Mark wasn't trustworthy, since he'd left the two earlier. Barnabus was right in that Mark was a good worker and deserved a second chance. Both men were men of God, and both were incredibly effective missions workers together. Nonetheless, there was no opportunity for peace if both went together. Therefore, they went seperately. They covered twice as many cities apart as together, and Mark went on to write one of the gospels. More importantly, they were at peace with each other again.
Unity of legal bodies isn't essential to Christianity. I disagree on too many points with John Paul to submit to his earthly authority and accept his beliefs as my own. However, I do recognize his spiritual authority as an elder in the church. I would happily worship with him at any time, and I will be happy to spend eternity in the New Jerusalem with him and many other Catholic leaders.
The unity that Christians need is the unity of the Spirit. We need to recognize that some churches do baptise differently, or hold different views on free will versus determination, or immersion versus sprinkling, or singing with instruments versus a capella. These differences may divide us by comfort or preference, but they do not divide us by salvation. In Jesus' sacrifice, we are united as one Bride to Christ, one church. That is where we are unified, not in one earthly authority or one set of "essential" doctrines. More Christians simply need to recognize and practice peace.