The professor of my introductory Christology class once told about a conversation he overheard in seminary. A seminary rector was relating his woes to an older rector concerning various abuses, academic neglect in particular, occurring in the seminaries around the country (Switzerland, but this story certainly applies universally). The older rector told him to concentrate on three core necessities in priestly intellectual formation which he then enumerated as: 1) Christology, 2) Sacramental Theology and 3) Ecclesiology.
As I've witnessed the posts and debates developing over the past few weeks, I've noticed that theological discussion, particularly in an ecumenical context, seem to radiate from the these three topics. They form a backbone (and sometimes a fault line) for what we are doing here and elsewhere. In light of this, what is the project of ecumenism? Well, in a sense we are faithfully attempting to arrive at a point where we can speak the same language. Do I mean that in a purely semantic way? No, such reductionism misses a fundamental point of language - communication and relationship. However, meaning can never be eclipsed by relationality, for indeed it is at the very heart of relationality as the self-revelation of being in communication. So, properly viewed, meaning and relationality are the two, inter-penetrating, sine qua non components of ecumenical dialogue when two are attempting to speak the same language.
Greg mentions the dialogue between the LWF and the Reformed and Josh (in the Comments) specifically mentions the agreements between the PCUSA and the ELCA. We might ask to what extent these dialogues 1) ground themselves in the three loci of theology mentioned above and 2) they are authentic dialogues in the sense of communicating in the same, or at least mutually comprehensible language of belief. The Lutheran/Roman Catholic Joint Statement on Justification seems a more acceptable model (although the project isn't as extensive and as such doesn't cover all the "bases" enumerated above).
Clint (in the Comments) raises a pertinent issue concerning the language of our dialogue -- "marks", "visible" and "revealed" are all terms a propos to ecclesiology, but how do they describe the life of the Church? To begin with, the life of the Church must never be considered as an entity apart from God. The Holy Spirit enlivens her during her earthly journey (consider the placement of Church within the article on the Holy Spirit in the creeds), and the Living Christ who abides in her through communion welcomes her in the fullest sense in the lasting communion of the resurrection. The three terms above, while wholly meaningful, describe a mystery that surpasses our understanding - a mystery that is to be consummated in the arrival of the Bridegroom. Our understanding, then, must have a real but also symbolic or analogical relation to the truth as known only to the Divine Mind. Christ, revealed by the Holy Spirit, must be the center of this understanding received in faith.
The mystery of the Church is fundamentally this - the mystery of humanity's union with God (Catholic Catechism 772). That this union has Christological and Sacramental overtones is apparent and appropriate. Clint applied Christological categories to sacramental theology and ecclesiology. While they must be qualified, they also must be explored (literally, analogically and spiritually/meditatively). Conversely the marks, visibility and revealed-ness are notions which belong in the greater context of theology to all three loci mentioned above. For example, what one has to say about the Holy Spirit as the revealer of Christ in Christology has an impact on pneumatology in the context of Sacramental Theology and Ecclesiology.
The Triune God and, in a special sense, Christ the God-man, as the source and terminus of unity, holiness, universality and apostolicity should be our starting point. Likewise, revelation and visibility must be understood fundamentally in what God in Christ and the Holy Spirit has revealed and consequently made visible. Sacramental theology and Ecclesiology proceed from this Christological/Theological foundation. Are we up to the task of sounding this out fully? In a word, no. No one is, in an absolute sense. Nonetheless, I propose that we faithfully seek the unity that enlivens the truth about God and his assembly. It is Christ who is at the center of meaning and relationality where two or three are gathered (Mt 18:20) in his name. And it is his wish that we follow when we pursue such unity (Jn 17:21).