A 4-Square Approach from the Anglicans

A primary source for the Quadrilateral was The Church-Idea, An Essay Towards Unity (1870) by William Reed Huntington (1838-1909), an Episcopal priest. He indicated the Anglican basis for an ecumenical "Church of the Reconciliation" in America should be acceptance of l) the Holy Scriptures as the Word of God; 2) the Nicene Creed as the rule of faith; 3) the two sacraments ordained by Christ himself (baptism and the eucharist); and 4) the episcopate as the keystone of governmental unity in the church. This "foursquare" approach became known as the "Quadrilateral." Huntington was the moving force behind its approval by the House of Bishops in Chicago.

The Chicago version of the Quadrilateral provides an ecumenical statement of purpose and introduction which states that the Episcopal Church is "ready in the spirit of love and humility to forego all preferences of her own" concerning things of human ordering or choice regarding modes of worship, discipline, and traditional customs. However, the statement of purpose warns that Christian unity "can be restored only by the return of all Christian communions to the principles of unity exemplified by the undivided Catholic Church during the first days of its existence."

Source: Episcopalchurch.org  -  "Chicago-Lambeth Quadrilateral", glossary entry for episcopalchurch.org
https://www.episcopalchurch.org/library/glossary/chicago-lambeth-quadrilateral