Since the times of the Early Church, Christians have been very discriminate about their prayer and in whose company they choose to pray. Already in the Apostolic Canons (Canon 65, for example), a document arguably dating back to the end of the second century, both lay people and clergy are prohibited from praying with heretics under the threat of excommunication. Apostolic Canon 45 mandates: “Let any Bishop, or Presbyter, or Deacon that merely joins in prayer with heretics be suspended…” Similarly, Canon 33 of the Council of Laodicea (ca. 363-364 A.D.) says that “one must not join in prayer with heretics and schismatics.” Yet common prayer is one of the central goals of the contemporary ecumenical movement, including the ecumenical dialogue between Orthodox and Roman Catholic Churches. Seemingly in defiance of the ancient canons, Catholic and Orthodox hierarchs have routinely joined each other in prayer, to the joy of the proponents of such practices and to the dismay of opponents.
Those working to make common prayer more common argue that the belief in one true God unites the different branches of Christianity and even those outside of the larger Christian community, thus all prayers ascend to the same divine destinations. Opponents often assert that heretics do not pray to the same God, but to the devil instead (cf. John 8:44). Thus, joint prayer is viewed as impossible (cf. 2 Cor. 6:15) or having the risk of accidentally addressing the wrong “authority”.
There is another point of view: if prayer is viewed not simply as locution or interlocution, but as an experience that is transformative for the devotee, even as a way or a mode of life, then it becomes easier to understand why those who doubt each other’s orthodoxy are so cautious about praying together. It is not the risk of accidentally addressing the “wrong” god that becomes central to warnings against praying with heretics, but the risk of being influenced by a way and a mode of life with which one may disagree, in other words, it is the risk to one’s spiritual health. (Imagine That… : Mental Imagery in Roman Catholic and Eastern Orthodox Private Devotion, Introduction)