What does the heresy of Ecumenism entail:
Three things:
(1) Praying with those in schism and heresy. Therefore, anyone who is in a True Orthodox Synod who communes outside of his synod is involved with Old Calendar Ecumenism.
For example, if a person in the Stephanos GOC Synod were to visit and pray with clerics or laity in any of the TOCs in communion with Archbishop Kallinikos (the former SiR, the ROCA parishes under Archbishop Agafangel, or the Romanians and the Bulgarians) or with any other TOCs such as the RTOC or ROAC, then that person would be guilty of Old Calendar Ecumenism and would be subject to excommunication from Archbishop Stephanos' synod.
(2) Praying with those who are in the New Calendar or who are in communion with those in the New Calendar such as all those jurisdictions in communion with the MP and the EP.
So, if a person in any TOC were to pray with or have communion with any member parish of World Orthodoxy (Antiochian Orthodox, Greek Orthodox Archdiocese under the EP, Moscow Patriarchal parishes, OCA, ROCOR parishes under the MP, Serbian Orthodox, etc.), then that would be considered to be Ecumenism, and that person would be subject to excommunication.
(3) Praying with those who are not Orthodox Christians, for example Roman Catholics, Anglicans, Protestants, Pentecostals, Jews, Muslims, Buddhists, pagans, etc.
Since the MP and the EP have been hosting inter-faith prayer meetings with Roman Catholics, Anglicans, Pentecostals, and Protestants, who have been having inter-faith prayer meetings with Jews, Muslims, Buddhists, pagans, etc., anyone with common sense can see that the water is very muddy.
Draw a Venn Diagram to see how far ecumenism has penetrated all our lives.