The word ecumenism, which comes from the Greek, originally means the whole inhabited earth. Ecumenical issues then concern the whole world. They are aimed at the unity of God's humanity, in a narrower sense the unity of the churches. The 20th century is described as an ecumenical age of coming together and unity. This unity does not mean unification, but unity in reconciled diversity.

Wiesbaden is characterized by the Protestant tradition. Until the middle of the 19th century, Catholic Christians from Wiesbaden had to attend church services in Frauenstein. This was to change when the first Catholic town church was consecrated in 1831. However, it collapsed shortly beforehand, so that the successor building, St. Boniface, could only be consecrated in 1849. After the Protestant town church went up in flames in 1850, the Protestants were given the opportunity to celebrate their services in St. Boniface's church, which had not yet been fully completed. Nevertheless, this ecumenical gesture was by and large more a case of the Protestant and Catholic churches working side by side rather than together. This began to change in the 1960s.

Today, Protestant and Catholic parishes invite each other to festivals, the governing bodies meet regularly, in some districts there is a joint parish newsletter, children's, youth, senior citizens' and educational work is carried out jointly in many places, ecumenical services are celebrated and people go on trips together. This is reflected in the fact that the boards of both churches meet twice a year at city level and are jointly responsible for many activities: e.g. the telephone and hospital chaplaincy, the Night of the Churches or the cooperation between the Wiesbaden-Rheingau-Taunus Caritas Association and the Diakonisches Werk in the social sector.

Since the 19th century, churches have also been built in Wiesbaden for Christians of other nationalities and denominations, such as the Russian Orthodox Church of St. Elisabeth and the Anglican Church of St. Augustine of Canterbury (also known as the English Church). The Christian denominational world of the German population also began to differentiate itself: the Peace Church of the Old Catholic Community was consecrated in 1900, followed by the Baptists' Zion Chapel in 1906 and the Methodists' parish hall in 1912. Other Protestant free churches such as the Free Evangelical Church and Pentecostal congregations expanded the Protestant spectrum. In the course of the recruitment of southern European workers in the 1950s and 1960s, an Italian, a Spanish, a Croatian and a Portuguese congregation were formed, which were soon integrated into the Catholic city church, as well as a Greek Orthodox and a Serbian Orthodox congregation.

Other Christian congregations have emerged since the 1980s in the course of global migration: e.g. various Baptist congregations of Russian emigrants, Presbyterian congregations of Korean Christians, an Orthodox congregation of Arabic-speaking migrants, a Syrian Orthodox, an Assyrian, a Polish Catholic, a Coptic and two Ethiopian Orthodox congregations. Christians of different denominations and national origins came together in the Arbeitsgemeinschaft Christlicher Kirchen Wiesbaden (ACK) in 1984.

In the course of worldwide migration and globalization, numerous independent small congregations, often of a charismatic nature, have also formed in Wiesbaden. People come from Eastern Europe, the Arab world, Africa, Asian countries or the USA. As the congregations are too small and have hardly any financial resources, they often ask Protestant or Catholic congregations via the ACK whether they can be accommodated there with their church services, youth and Bible study groups, among other things. Many congregations of the two large churches now have such host congregations. World Christianity, with its Orthodox, Catholic, Protestant, charismatic and independent churches, is becoming increasingly represented in major German cities such as Wiesbaden.

Through their ACKs, the Christian churches in Wiesbaden and elsewhere are making a significant contribution to the integration and stabilization of civil society. They recognize the signs of a globalized world and develop ecumenical partnerships with communities or regions in other countries, either alone or with others, such as the Protestant Deanery, which has entered into an ecumenical partnership with the Evangelical Lutheran Diocese of Wroclaw, Wiesbaden's Polish twin city, and with the United Church of Christ in the American state of New York. In the 1980s and 1990s, Europe's Christian churches increasingly turned their attention to issues of international justice, peace and the integrity of creation. As a result, numerous one-world, environmental and peace groups also emerged in Wiesbaden.

In a long process, this European movement gave rise to the Charta Oecumenica, which was signed by the Conference of European Churches and the Council of European Bishops' Conferences in 2001. In 2007, the ACK Wiesbaden adopted this document after a lengthy discussion process.