The thing that makes Satan such an effective deceiver is how well he can mix truth with a lie to make it believable. That's what he did with Eve in the garden and it's what EVERY false church/teacher does. Could the Orthodox churches have more truth than other divisions? Certainly but neither have the whole unadulterated truth. That's a problem when it comes to the Lord. He never leaves his children in such ignorance, therefore they're not his regardless of how much truth they have. It's to no effect.