The word ecumenism comes from a family of Classical Greek words: oikos, meaning a “house,” “family,” “people,” or “nation”; oikoumenē, “the whole inhabited world”; and oikoumenikos, “open to or participating in the whole world.” Like many biblical words, these were invested with Christian meaning. The word oikoumenē describes the place of God’s reconciling mission (Matthew 24:14), the unity of the Roman Empire (Luke 2:1) and of the kingdoms of the earth (Luke 4:5), and the world destined to be redeemed by Christ (Hebrews 2:5). The vision of one church serving God in the world came to reflect a central teaching of the early Christian faith.

In later centuries the word ecumenical was used to denote church councils (e.g., Nicaea, Chalcedon) whose decisions represented the universal church, in contrast to other councils that enjoyed only regional or limited reception. The honorary title of ecumenical patriarch was given to the Greek Orthodox patriarch of Constantinople in the 6th century because his see was located in the capital of the oikoumenē and his leadership was accepted as primus inter pares (first among equals) in the faith and mission of the whole church. The Apostles’, Nicene, and Athanasian creeds are called ecumenical because they witness to the faith of all Christians. Since the 19th century the term ecumenism has denoted the movement of the renewal, unity, and mission of Christians and churches of different traditions “so that the world may believe.”

Ecumenism is a vision, a movement, a theology, and a mode of action. It represents the universality of the people of God and affects the way Christians think about their faith, the church, and the world. Ecumenism is a long process that draws Christians together, uniting their life and mission and bringing the Body of Christ and the human community closer to the fulfillment of God’s purposes. Those involved in ecumenism participate in ideas, activities, and institutions that express a spiritual reality of shared love in the church and the human community. Ecumenism is characterized by the work of officially organized ecumenical bodies, the confessing and witnessing of Christians in local places, and the spirituality and actions of those who live together in love and prophetic proclamation. Far more than a program or an organization, ecumenism is, according to the British ecumenist Oliver S. Tomkins, “something that happens to the soul of Christians.”

Any unity worthy of this vision cannot be identified with political or spiritual coercion, strategies of dominance or superiority, calls for “a return to the mother church,” or expectations of monolithic uniformity or a super-church. The weapons of faith are not those of force or intolerance, and neither can divisions be overcome nor authentic unity manifested by syncretism, a least-common-denominator theology, or a casual friendliness. Ecumenism accepts the diversity of God’s people, given in creation and redemption, and strives to bring these confessional, cultural, national, and racial differences into one fully committed fellowship.

Ultimately, the purpose of ecumenism is to glorify the triune God and to help the one missionary church to witness effectively and faithfully among all peoples and nations. In the second half of the 20th century, Christians began confessing new dimensions of this vocation, especially in relation to what divides the churches. Progress was made on historical theological issues that have divided Christians through the centuries—baptism, the Eucharist, and ministry. But equally divisive among Christians are the divisions of the human family: racism, poverty, sexism, war, injustice, and differing ideologies. These issues are part of the agenda of ecumenism and bring a particular context, dynamic spirit, and urgency to the pursuit of Christian unity as well as of justice and peace. The church’s unity becomes essential for the renewal and unity of the human family. Through its unity the church becomes a sign, the first fruits of the promised unity and peace among God’s peoples and the nations.