George Calixtus was quite the optimist. The seventeenth-century Lutheran scholar is best known as the father of modern ecumenism. He lived in an age of divisiveness, when denominations and nationalism wedded together to make differences in theology warrant not only disunion but violence. His was a world that justified pessimism. Calixtus, however, longed for unity and thought it a real possibility. His basic idea was that if Christianity could be boiled down to commonly agreed upon essentials then the church could settle on a tolerance for diversity of opinions regarding non-essential doctrines. It was a classic example of unity within diversity.
Of course, one could argue that Calixtus’s propositions made him the least popular theologian of the seventeenth century. At first Lutherans accused him of being Catholic, while Catholics found him overly compromising. Then as he tried to include the Reformed Church, Lutherans accused him of being a covert Calvinist, while the Calvinists just thought he was a heretic. Lucky for Calixtus, he had good friends who protected him throughout his career. 
His example sparks many questions for me; the foremost among them: Is it possible to construct a theology of unity within diversity without ending up as the most hated person in the room? This may seem entirely unrelated at the moment, but my answer to my own question is covenanted relationships.
Take, for example, the epic demonstration to faithful relationship we call the Old Testament. God makes repeated covenants with his people, who repeatedly provide reasons for him to break the covenant. God, however, is stubbornly, graciously, and sacrificially committed to maintaining the relationship. They worship other gods, and still he reaches his hand of fellowship to them. They vacillate between straight-up denial and self-serving devotion, and still he longs to dwell among them. They yearn to be like the peoples who do not have a relationship with God, and still he journeys with them. They exploit the poor among them and ignore justice, which finally compels God to withdraw his presence from the temple (Ez. 10:1-22; cf. Is. 1). Even then, God commits himself to the project of reconciliation and restoration.
But you and I, and our other brothers and sisters? Well, disagree with me about the color of the carpet and I’m outta here. Question my nation and you’re toast. Challenge my understanding of who Jesus is and I’ll stop listening. Dare to argue that I might have something in common with (insert your tradition’s least favorite group here) and I’ll hate you more than those heretics. Our understanding of the relationship between believers is tenuous at best. While God endures millennia of rejection, we can’t stand to sit at separate tables for a potluck. It seems to me that Calixtus’s challenges had little to do with theology and a whole lot to do with a historically low view of believer-to-believer relationships.
As I mentioned above, part of Calixtus’s milieu was the rise of nationalism. It’s no surprise that Lutheran Germans hated the Catholic Spaniards—Germans distrusted Spaniards generally. Doctrinal traditions were good rallying points, items of differentiation. Through the lens of biblical covenant we arrive at an irony: Germans held together in covenanted relationships during the Thirty Years War, for instance, but couldn’t imagine a covenanted relationship with believers of as similar a stripe as Calvinism. We could say that the same is true within the borders of our own country, where the strength of Republican-to-Republican or Democrat-to-Democrat relationships feels almost covenanted, while the Mainline-to-Evangelical relationship feels non-existent. So while Calixtus constructed a theological answer to disunion, non-theological forces were (and are) codifying divisions. Perhaps his was actually a classic case of treating the symptom and not the cause.
All this brings me back to street-level covenant. I pastor a small neighborhood church committed to unity. No one signs an actual covenant. We don’t even use the language of covenant. Instead, we tend to talk about listening to one another and creating space for each other. We talk about respecting what the Spirit is doing in each and every person. And we dwell a lot on the idea that what we’re uniting behind is singularly a common trust in our life-changing Jesus. If you believe in Jesus, in effect, we want to have a covenanted relationship with you. By grace we find ourselves woven into a patchwork of backgrounds, points of views, convictions, and problems.
However, one thing that strikes me in my experience is that there are a lot of people who have zero interest in that kind of commitment to each other. Every few months we meet a person who is looking for a church who brings along a pet project (often theological in nature). They enjoy our songs and sermons and potlucks for a month or so and then they sit down with me. The only thing they want in that meeting is to figure out if I’ll assent to their pet project. The challenge for me is to keep myself committed to them as a follower of Jesus without leading them to think that there is any possibility that I’ll convert our congregation to messianic Judaism (for example). I typically say something like, “I appreciate your desire to serve Jesus and what’s percolating in your spirit, but I’m not sure that I’m in a place where I agree with you and here’s why . . . .”  I desperately want to be filled with a spirit of deep commitment to them. The opposite is nowhere near true. It often isn’t long before I hear that they’ve taken their project to the attention of another pastor in town. I shudder to assume such knowledge, but it appears that they are promised to their project and not the Church. Their interest is conformity and diversity feels like the antithesis of that calling.
So where does that leave us? We can theologize and dialogue all we want. We can even walk in peace and solidarity, but as long as social and political allegiances trump our membership in the Gospel-created-people we will find it far too easy to cast one another aside. Unity within diversity just won’t happen without a renewed commitment to one another; a covenant that transcends our cultural categories. Every week my ragtag, varied congregation makes optimism a feasible option for me. Then again, every week some internet-buzz feeds me an example of a divisive tirade that casually dismisses sisters and brothers. Pessimism waxes.
Calixtus’s general theological framework, I think, is still viable. However, where we must build upon it is in cultivating a steadfast commitment to graciously journey with each other through pet projects, selfishness, elections, rebellion, arguments, potlucks, international crises, controversies, and theological differences.
Unity within diversity will require that we be a people of covenanted relationships.
— You can see Calixtus’ sparse Wikipedia page here or there’s a slightly more thorough introduction to him in Justo Gonzalez’s The Story of Christianity, Vol. 2.
 We want a great, trusting, and life-giving relationship with you even if you don’t believe in Jesus, but that’s a different topic.
 So far I’ve never lost it, but I’ve been very close.