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Veteran
I'm not religious now but from a distant memory it would have to be the Orthodox first since it was the Romans that put him on the cross.
The church split in two when the Roman empire split in two. The Eastern Roman empire became Byzantium, and the official church of Byzantium is what we call the orthodox church. The official church of the Western Roman Empire is what we call the catholic church.
I think the Roman bit that the protestants attach to the name confuses things. That Roman refers to the city, as seat of the pope, not the empire.