Why wouldn’t we? They’re validly ordained priests. I had a professor, that is an Eastern Orthodox priest, and even the Evangelical Protestants that have a hang up on calling priests father, still called him father. A lot of people just have great respect for him. As far as I know, calling an Orthodox priest father is fine for a Catholic
Pope Benedict addresses the Patriarch of Constantinople as “Your Holiness”
If you search through the other volumes of AAS (the official Acts of the Holy See) you’ll find hundreds of such examples. That one just happened to come up at random.
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