"Unite" has always been one of my favorite songs by the O.C. Supertones, partly because it's an awesome song, but also because of its message of sparking unity among Christians. Here is an excerpt:
O yeah, I got a beef with the fence-sitters
Tares among the wheat, the cop-outs, the quitters
Cut from the branch fruitless, no good,
Only one use and that's firewood
Pay no mind to the generation line
Forsake your sect and be color blind
The problem's not Hollywood, the problem's not Washington
The problem's a weak divided church of schizmed Christians.
One of the last things Jesus did for His disciples was pray for them, and a reoccurring phrase was that they would be one. He also prays the same thing for all of the future disciples, our unity ultimately being the thing that lets the world know He was who He said He was.
My prayer is not for them alone. I pray also for those who will believe in me through their message, that all of them may be one, Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me. I have given them the glory that you gave me, that they may be one as we are one—I in them and you in me—so that they may be brought to complete unity. Then the world will know that you sent me and have loved them even as you have loved me. (John 17:20-23)
So clearly, unity is of the utmost importance. So why are we so divided, and how do we unite again?
I'm sure the answer to that first question is multi-faceted, spanning eras of Christian history and doctrine, schisms and the like. Here is a good article on the subject, which sheds much light on the situation and current movements towards church unity. In my humble opinion, and based on my observation, it seems most Christian breaks occur over differences in doctrine and interpretation of scripture. And as I imagine what it would take to truly unite a church, I can't imagine a world where we don't disagree on something.
Therefore, as God's chosen people, holy and dearly loved, clothe yourselves with compassion, kindness, humility, gentleness and patience. Bear with each other and forgive whatever grievances you may have against one another. Forgive as the Lord forgave you. And over all these virtues put on love, which binds them all together in perfect unity. (Colossians 3:12-14)
The fact is that we're not going to agree on doctrine. But how much emphasis should we place on doctrine? I know for myself that the doctrine I dogmatically held fast to ten years ago is not the same doctrine I adhere to now. I don't mean that in the way of major theologies, pillars of the Christian faith (i.e. - Jesus is God in flesh, died and rose again), but rather the idiosyncrasies of our faith that are likely what lead to division. I'll give you an example: when I was a new believer, I had a friend who was interested in the faith, but asked me, "Do I need to stop having sex to be a Christian?" I replied with, "yes." His face went downcast as he was not willing to give up such a pleasure, though he was clearly interested in a relationship with God.
Were I asked that same question today, I would have a different reply,  something along the lines of, "Don't worry about that. God wants you right now, as you are. Accept that and we'll go from there." I still don't believe extra-marital sex is okay (based on scripture), but I also believe that God doesn't ask us to clean up before we approach Him. From there, if God wants to convict my friend about his sexual behaviors, He will, and I trust that. God might even ask me to call him out on it. But my response so many years prior may have kept a kid from entering the Kingdom of God; asking him to change his behavior prior to letting God change his heart.

I write all that as an example of how my maturity in faith has led to changes in the way I interpret scripture and interact with others. I know that in ten more years I'll have slightly different views as God brings me closer to who He wants me to be. And so, to be dogmatic about certain aspects of doctrine is dangerous. (I know I will meet some opposition on this, but that's why I write "dangerous" and not "wrong." Sound doctrine is indeed important and I don't want to diminish that, but if your doctrine is dividing the church, maybe you are handling it improperly.)
Back to that verse above: What does it mean to "clothe yourself" with these virtues and why do we do it? Simply put, you take clothes on and off. You can do the same for virtues. Some situations require more kindness than others. Some patience. There are times you may not need any of these, but I don't think Paul's wording of "clothe yourself" is unintentional; and I also can't ignore that these suggestions are sandwiched in the middle of a passage about church unity, and followed immediately by, "Bear with each other and forgive whatever grievances you may have against one another. Forgive as the Lord forgave you." (v.13)
So in our pursuit of church unity, we have to be ready to disagree, but willing to clothe ourselves with various virtues to maintain peace. Tangibly speaking, clothes are necessary for social interaction, and also what others see of us from the outside. Sometimes we wear them when we don't feel like it, others we wear because the weather calls for it. I don't like shoes, but I wear them because people aren't comfortable with me being barefoot all the time, and also because I don't want dirty feet. It doesn't change the fact that I don't like it, but the benefits outweigh the cost. (silly example, but true...)
The key to this whole part, though, is forgiveness.
Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you. (Ephesians 4:32)
We MUST forgive. It is not an option (Matthew 6:15), not only for our sake, but for the sake of the church. Forgiveness is the pivot point on which unity rests. Without it, we are unbalanced and incomplete, our virtues  emptied of much of their effectiveness.
Just two verses later in Colossians, Paul admonishes us to "Let the peace of Christ rule in your hearts, since as member of one body you were called to peace." (Colossians 3:15a) To understand what this means, imagine what the opposite of the peace of Christ would look like? Anger, rage, malice, slander, filthy language (Colossians 3:5), quarreling, jealousy, outbursts of anger, factions, gossip, arrogance, disorder (2 Corinthians 12:20b). Paul says, "Let the peace of Christ rule," as in, we have a choice to let it rule or not.
"Rule" here means "act as an umpire." Peace is to be the umpire for our activities together. When we are drifting from the rule of peace, the umpire calls, "Foul!" and the play is over. Without it, we have chaos and a body divided.
Though we have a choice in the matter, our calling is clear: "...as members of one body you were called to peace." (Colossians 3:15b) Why? A body is simply not functional if it is divided against itself.
Every kingdom divided against itself will be ruined, and every city or household divided against itself will not stand." (Matthew 12:25b)
This is the same as an autoimmune disease: the body becomes confused as to who the enemy is and starts attacking itself. Is this the current state of the church? Instead of vibrant and alive, are we attacking ourselves? That's likely a bit dramatic, but worth considering.
  1. When you meet a Christian who thinks differently than you about a certain matter, do not consider that person less-Christian, but rejoice that he or she is on the same journey as you, just likely on a different timeline. I've been on both sides of this, judging and judged. Neither is positive.
  2. Pursue projects that unite Christians: It always bothered me that the Baptist churches back home only worked with other Baptist churches (not cornering Baptists, that's just where I went growing up). Meanwhile, I had friends at a great Episcopalian church across town that was doing great work, and that I would have loved to partner with. I'm sure there are logistical reasons for this, but I've seen cross-denominational work turn out wonderfully also.
  3. Be open-minded: If you come across something that challenges your faith, go to God about it. Seek scripture. Bring it up with other trusted believers. The challenger may be wrong, but maybe he or she is right, and you need to grow. Either way, don't let it ruin a relationship that could otherwise be beneficial for you, them or the Kingdom. As long as it doesn't lead down unhealthy roads, there may be times to "agree to disagree" for the sake of bigger things.
Note: Take this post with a grain of salt -- I'm still learning myself. Whether you agree or not, I'd love to have some conversation about church unity in the space below and see what people think.