Eastern and Oriental Orthodoxy
It is a puzzle, and Peter, like Abba Seraphim, explains it terribly well. There is much that could be said on another occasion, and there is a lot on the BOC website that helps us through this. But having debated this elsewhere, what saddens me is the spirit of narrow exclusivity evinced by some EOs. It may be some OOs do, but maybe I have been lucky and not seen that. But the Copts, perhaps because they have never been an Imperial Church, seem much more eirenic, and whilst holding to the Orthodox Faith, do not seem to insist that God is their Father and their Father only.
It does seem to me that if one goes back to Chalcedon, there were real differences, based on real issues which were not understood in the same way by the different sides involved. But, essentially, this was a family quarrel, which, like all such, makes Our Father sad at our fallibility; fortunately, He is the ultimate loving Father and He is always willing to forgive those who repent. We all have much to repent, and he who says he is without sin speaks folly.
The Orthodox Unity website at <!-- m --><a class="postlink" href="http://www.orthodoxunity.org/">http://www.orthodoxunity.org/</a><!-- m --> gives more than enough evidence that without compromise on essentials, movement towards unity is possible - and it is so sad that Christians with so much in common (more than some of them know) remain sundered.
But that does not stop us being brothers and sisters in Christ - even if some don't acknowledge the family resemblance!
In this is love, not that we loved God, but that He loved us and sent His Son to be the propitiation for our sins. (1 John 4:10)