In his endeavours to relate to members of other religions, some Orthodox felt that Archbishop Iakovos went entirely too far, that he violated the [[canons]] and betrayed the faith by praying with [[heresy|heretics]]. Perhaps one of the more famous instances of an Orthodox hierarch rebuking Archbishop Iakovos was Metropolitan [[Philaret (Voznesensky) of New York|Philaret]] ([[ROCOR]]) writing an [http://www.orthodoxinfo.com/ecumenism/philaret_iakovos.aspx open letter] on the occasion of Iakovos's participation at St. Patrick's Cathedral in the "Week of Prayer for Christian Unity
," and the [http://www.orthodoxinfo.com/ecumenism/doxology.pdf "Ecumenical Doxology"] in the Greek Cathedral of the Holy Trinity in New York City.
During his tenure as archbishop, Iakovos drew a great deal of criticism from those especially concerned about his activities pertaining to [[ecumenism]]. His own definition of ecumenism was as follows: "Ecumenism is the hope for international understanding, for humanitarian allegiance, for true peace based on justice and dignity, and for God’s continued presence and involvement in modern history" (ca. 1960).