Catholic Christians understand the Unity of the Body of Christ to be a primary concern for all Christians. I have written before that I believe many folks to be "inappropriately scandalized" by the fact of Christian division. If you are a baptized person, it should bother you. Lots. At least enough to do something about it.
For evangelical protestants, the unity of the Church looks like people getting along and sharing prayers and ministry. This is as true and important, as far as it goes. I think many evangelicals would also say that doctrinal unity - specific assent to particular theological points - is an important aspect of unity (perhaps the most important) and is a prerequisite to shared mission in many cases and perhaps a prerequisite to sharing a life of prayer and friendship.
Catholic Christianity is concerned with doctrine, mission, and getting along, but for us, it looks very different - it looks like questions of church order. Of course sacramental validity fits in there as well. For evangelicals, the thing we understand as church order often seems arbitrary, but I'll try to explain.
While it may seem to many that the ancient church consisted of a "mixed economy" of alternative and competing Christianties (much like today's Protestant milieu), from the very beginning churches were differentiated by geography, not by their particular version of the Faith. The "local church" was the assembly of all Christians in a particular place, not a small "congregation" grouped by preference or affinity. Within the first several decades after Christ's Ascension, an order that historians call the "monarchical episcopate" had emerged - instead of the local church in each city being ruled by a college (or council) of presbyters, there emerged one overseer, or bishop, from that college. He was understood to present Christ as shepherd to the Church, and became a focus for unity of the wider Church. It also quickly became important that these bishops have the right relational pedigree; in an age where teachers of alternative Christianities kept cropping up and claiming special revelation or access to secret teaching that had been passed down from Jesus through some shadowy characters in a fashion that was impossible to confirm, it was important to know that a bishop had been discipled (apprenticed or formed in the Christian faith) by someone who was known to be a close associate of the Apostles. The bishop's power and prerogative to ordain was considered to be derivative of the authority Jesus invested in the Apostles, and when priests acted in Christ's name to preside at the Eucharist and to grant absolution of sin, they were understood to derive their authority from their bishop. It was also understood that these bishops and therefore their priests would have been formed according to the Rule of Faith, which later became known as the official creeds of the Christian Church - I mean specifically the so-called Apostles' Creed.
When a community could claim that pedigree, one knew that the community in question professed and practiced the true Christian faith, and that this was a community that Jesus transformed by his ongoing action through the sacraments.
So when we consider the question of Christian unity, we believe it to have several expressions:
1. Is this a community of Christians that derives from apostolic continuity, or did it spring up from someone else's peculiar Bible reading, or particular version of Christianity? Unity in the Church requires continuity with apostolic Christianity.
2. Do the bishops of particular communities recognize one another as teachers of the apostolic faith, who have been consecrated in the apostolic succession?
3. Does the community profess and teach the Bible according to the Creeds?
After answering these joint questions of doctrine, church order and sacramental validity, then we concern ourselves with what it means to get along well with one another, and to recognize one anothers ministries as Christian communities, and start agreeing together about what it means to be Christian people.
you’re perfect just the way you are
6 months ago