Why is it not permitted to have joint prayer?

"My dear ones, there are Canon of our Church that forbid joint prayer with heretics. Of course, those who support joint prayer talk about love. Yes, but love, as the New Testament points out must be in Truth... and Truth means Orthodoxy... Why is it not permitted to have joint prayer? Because this prayer is not in Truth... I will remind you of what I have told you countless times: ecumenism is deceitful and it progresses in stages. Its first step is to achieve the [false] union of Christians, and then to achieve the union of Christians with Muslims and afterwards with the idol-worshipers. They are deceitful, they proceed progressively, step by step."

-- Fr. Athanasios Mitilinaios

Additionally, from Saint Ephraim of Katounakia: "Ecumenism does not have the Holy Spirit, but the unclean spirit."

See the reel here: