According to the WCC constitution, "agreement with the basis upon which the Council is founded" is a precondition for membership. Adopted by the inaugural assembly (Amsterdam, 1948), the original basis read simply, "The World Council of Churches is a fellowship of churches which accept our Lord Jesus Christ as God and Saviour." It had been formulated at a meeting in Utrecht in 1938 of the committee of 14 appointed by the Life and Work (L&W) and Faith and Order (F&O) conferences.

"Fellowship of churches" had by 1948 become part of ecumenical terminology. The 1920 encyclical of the Ecumenical Patriarchate of Constantinople had proposed "a koinonia of churches". Although the English word "fellowship" lacks the rich biblical nuances of the Greek original, it does affirm the reality of a unity that is "given" and "previous", and not just constituted by human decisions, and implicitly rejects the WCC as a potential "super-church".

"Which accept our Lord Jesus Christ as God and Saviour", some claim, finds its source in the 1855 basis of the YMCAs, and later of the World Young Women's Christian Association (1894) and of the World Student Christian Federation. More directly, invitations to the first world conference of F&O were addressed to churches "which accept our Lord Jesus Christ as God and Saviour".

Some in both liberal and conservative circles expressed dissatisfaction with "Jesus Christ as God and Saviour". Unitarians and the Society of Friends did not want to be committed to a definite doctrinal formula. To the more orthodox the phrase did not adequately affirm the humanity of Christ. It has "a heretical flavour which would have led to its rejection by any one of the ecumenical councils" (William Adams Brown).

In his 1938 explanatory memorandum on the WCC constitution, William Temple, who had chaired the Utrecht meeting, drew out the two main implications of the basis as formulated. First, the fact that the WCC is a fellowship, not a federation, of churches means that it cannot exercise any constitutional authority over the member churches. Second, the Council stands on faith in Jesus Christ as God and Saviour - in essence "an affirmation of the incarnation and the atonement". But the basis is "not a credal test to judge churches or persons"; the churches will have freedom to interpret that faith in their own way.

From Utrecht on, some have argued against any basis. It could introduce an element of ecclesiastical judgmentalism and so corrode the koinonia. Others would prefer the Nicene or Apostles' Creed as the basis.

Although the Amsterdam assembly viewed the basis as "adequate for the present purposes" of the WCC, it endorsed the need "for clarification or amplification of the Christian faith" within the Christological framework which the assembly had affirmed. A later study by the central committee concluded that there was no need to change the basis, though it was necessary to explain its meaning and also make clear that the incarnation and the Trinity were implicit in it. Accordingly the second assembly (Evanston 1954) accepted a description of the purpose and function of the basis: "less than a confession" but "much more than a mere formula or agreement". The basis showed the nature of ecumenical fellowship, provided an overall orientation for the Council's work, and indicated the general range of fellowship which the member churches sought to establish.

After Evanston another study led the central committee to present a new basis at the third assembly (New Delhi, 1961); it was adopted with 383 votes in favour, 36 against and 7 abstentions. It reads: "The World Council of Churches is a fellowship of churches which confess the Lord Jesus Christ as God and Saviour according to the scriptures and therefore seek to fulfill together their common calling to the glory of the one God, Father, Son and Holy Spirit."

The re-formulated basis incorporates five changes. "Confess", versus the earlier "accept", suggests commitment and emphasizes the experience of togetherness in fellowship. "The", not "our", with "Lord Jesus Christ" is less restrictive and points to the universality of Christ's lordship. "According to the scriptures" to an extent meets the criticism that the earlier version tended towards Docetism or monophysitism and, at the same time, affirms the place of the Bible in the ecumenical fellowship. "And therefore seek to fulfill together their common calling" adds a dimension of dynamism to the understanding of fellowship and also underlines the ontological priority of what God in Christ has already accomplished. The final doxological formula sets the Christocentric affirmation in a Trinitarian setting, makes the basis totally acceptable to the Orthodox and adds a celebrative element to the fact of and aspiration for unity.

Most of the assembly delegates who took part in the discussion were of the opinion that the new basis was in full agreement with the Trinitarian doctrine as formulated by the first two ecumenical councils and in the Nicene Creed, and that it made more explicit the evangelical and scriptural rationale of the ecumenical movement. But there were also critical voices. They feared that, in going beyond the essential Christological criterion for membership, the WCC was moving in the direction of confessionism, or that any expansion would set a precedent for still further additions until the basis became "a burdensome doctrinal statement". Other critics imagined that the new basis would block any future revision and leave uncorrected "the one-sided monophysite character of the original basis".

The 1961 basis has endured. It continues to sufficiently define the WCC's nature. But the one sentence and each of its key expressions are not static abstractions. They are coloured by the horizons which over 50 years of reflective experience have developed: the Lord Jesus Christ as Lord and Saviour, the scriptures as understood, prayed and witnessed, the fellowship of churches and their common calling, even the glory of the Triune God. None of these realities is quite experienced and understood as it had been by the fledgling member churches in 1948. In fact, this developmental continuity creates a "basis beyond the Basis", evidenced in the central committee's lengthy policy statement proposed to the eighth assembly (Harare, 1998): Towards a Common Understanding and Vision of the WCC

 T. K. Thomas, “WCC, Basis of,” in Dictionary of the Ecumenical Movement, 2nd ed., ed. Nicholas Lossky et al. (Geneva: WCC Publications and Grand Rapids: Eerdmans, 2002), 1238–39.