This room is for discussion concerning issues related to what is commonly described as the "Traditional Catholic" movement in the Roman Rite and any issues related to the practices of Eastern Rite Catholicism.
Saint Athanasius is counted as one of the four Great Doctors of the Church.
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There is no concelebration in the Orthodox Church so it is unlikely you would ever see that. However you could see a Mass celebrated by one other the other where the Pope or Patriarch acted as deacon.
You mean intercommunion. I was referring to the restoration of full communion.
Also, the Orthodox definitely DO practice concelebration in the Divine Liturgy