When Can Unity be a Priority?

I've been pondering what the Church would look like with unity added. I've been thinking this because I know that it was one of the primary subjects of Jesus' prayer in John 17:20: I do not ask on behalf of these alone, but for those also who believe in Me through their word; that they may all be one; even as You, Father, are in Me, and I in You; that they also may be in Us, so that the world may believe that You sent Me; it is again emphasized throughout Ephesians 4:1-16, that we as Christians be unified. And yet, our churches throughout our American culture, and maybe worldwide, are so vastly divided I can't even imagine what they would look like if they functioned as if under one banner, one flag...one God...as they all would doubtless claim to do. Unity doesn't even seem to be a realistic priority in the Church as a whole, and it confounds me.
Imagining the global Church as unified is almost as unimaginable as heaven, and this to our shame. It should be imaginable; it should indeed be a reality; it should be something on which we are experts! It should be basic to our faith, assumed rather than dismissed. It should...but it's not. I'm realizing just how vastly divided our factions of faith are. We believe different things; we distrust each other's belief systems; we contradict each other; we shoot each other down and alienate one another. This is so contrary to God's wishes; aside from the strife itself, it sends a message to unbelievers that we are no better than they are.
So what would the Church look like if we were unified? (I'm not saying I know, alas!) Would we have one big mega-church per city? Would we still have our various congregations, but have all the congregations working and fellowshipping together? Would there be twice as many Christians because the unbelievers wouldn't see this message of strife and confusion, but would be drawn to this vast loving entity that so clearly imitates Jesus Christ? Would we all believe the same thing? Would there even be any cults? (If there were, I think they'd be more recognizable as such, because the true Church would make such a strong statement of what Christlikeness would be. Cults would be less attractive because people wouldn't be disenchanted with strifes, divisions, hypocrisy, and other forms of sin that prevail in the Christian faith.) I honestly don't know one answer to these questions from having seen it.
The closest thing that I've seen to a unified Church is the fellowship of the country churches in the very rural area where we've been attending over the last year. They go out of their way to link arms in various opportunities for fellowship among them, and I haven't seen among them the competitiveness and mutual suspicion that otherwise tend to be so prevalent. The pastors from various churches meet together every month; one church has VBS every summer, ours has a New Year's Eve party, another has a Grand Prix that they open to the kids in the community, we've come together to hand out pamphlets that list the offerings of the various churches to new residents in the area, and our pastor has mentored a member of his congregation to become a pastor in one of the other churches. To an extent, it's a tiny picture of what the Church should be all over the world. And in 18 years of being a Christian, I've never seen such cooperation among churches until this last year.
What is it that causes this lack of unity that prevails even after 2000 years? I can see that some of it might be the natural human tendency to want to be superior, to feel that we can look down on someone else and say our church is better, that we have a better grasp, to imagine that God approves of us more, that the others have fallen for a sub-standard and lack for some vital element of faith, as well as an insecurity regarding the size of our church in comparison to others. It might also be that we don't all read the Bible as much as we should, or with humility and desire to interpret it according to God's view and not our own, so we won't come up with endless variations on what it says, causing disagreement among us. It may also be that we don't pray enough--any of us--for unity in the Church. Maybe all of us are some of the problem, in that we're not enough of the solution.
In my own Christian faith, I have experienced a subtle adjustment to the culture of each church we have attended, and when I look back I am struck by just how much differently I have believed in times past, even since accepting the fact that Jesus was my Lord and Savior. These days I am examining various elements of my beliefs and trying to ascertain which are consistent with biblical truth rather than just the biases and assumptions that I have been taught through the years. It's similar to the examination I did when I came to Christ and had to disassemble, evaluate and discard many facets of what I'd thought before. I think that I assumed that the more unfamiliar the ideas were that I encountered as a new Christian, held by the Christians I was meeting, the more they must be Christian rather than secular. As I go along, though, I find that many concepts that I assumed were Christian are more just a human than biblical concept, even though oftentimes people who hold their individual views attempt to justify them with various Scriptures. Perhaps I have been too lackadaisical in examining these beliefs along the way, checking them more clearly against the Scriptures, even though that is a practice that I try to keep with some diligence. Scriptures are so easy to twist to one's own liking even without the conscious intent to deceive; all the more reason to beware and test everything. Psalm 146:3 says, Do not trust in princes, In mortal man, in whom there is no salvation; this is also reiterated in Ephesians 4:14: As a result, we are no longer to be children, tossed here and there by waves and carried about by every wind of doctrine, by the trickery of men, by craftiness in deceitful scheming; but speaking the truth in love, we are to grow up in all aspects into Him who is the Head, even Christ... I don't think these verses only warn that we beware of men who are trying to deceive, but they take into account that no mere human has a monopoly on truth or on wrong ideas. We need to examine every idea presented to us, no matter how trustworthy its human source may seem, against the Scriptures.
The prevailing "various winds of doctrine" are the cause and the result of a disunified Church. The differences between believers are sometimes subtle and tricky. Sometimes they are vast even within one congregation. If we could together stop focusing on the differences, start focusing on what God's word says, and let everything be done in love, building up the Church rather than puffing up selves within it, I think the Church would grow, God would be pleased, fellowship would increase even between congregations because it would be so much more attractive, iron would have more chance to sharpen iron (and therefore we would be more consistent in our beliefs)...and let me tell you, I'd sure be a lot less confused!
So what can I do to change it? I can pray that unity would increase; I can discourage others from belittling other churches and other believers; I can come up with ideas for fellowship between churches; I can reach out to other believers in spite of differences; I can avoid thinking on differences and isolate my thinking to the things that unite, and see what we can do to love one another more. God's first most important commandments were to love Him with all our heart soul mind and strength and love our neighbor as ourselves. I don't remember much emphasis in the Bible about how we are to check out the differences between us and camp on them.