I think one of the most sensible things said in the realm of Ecumenism for quite a long time was the welcome given by the late Patriarch Alexis of Moskow to Benedict XVI's liberation of the Old Roman Ordo Missae."The recovery and valuing of the ancient liturgical tradition is a fact that we welcome positively".
I always feel uneasy about Western Christians who adopt a few of the sexier externals of Byzantine devotion and feel very pleased with themselves about it (Byzantine customs with regard to fasting are not commonly among the elements they appear to borrow). Likewise, I get no pleasure at all from the thought of footloose Byzantine Christians believing that their Christianity is incomplete until they have gone shopping in my tradition. Each lawful tradition is in itself a wholesome and holistic entire way of living the Christian life.
The real Ecumenism is: that each of us should drink ever more deeply in the pure fountains of our own tradition. As we come to know our own way to Christ better and better, we shall discover surprising things about our oneness in Christ*.
As Patriarch Alexis went on to say, it was purely through its rootedness in the Byzantine tradition of lived worship that the Russian Church was enabled to survive the twentieth century atheist persecutions. It is in commitment to the life of each tradition that the Lord, and the strength he gives, are found.
*To take as an example a small matter of detail: when recently, after carefully investigating the Roman tradition, I wrote about the Diaconate as a cultic, not philanthropic, Order, some Western comments did not grasp my point: ".... it's surely both .......", they cried, perhaps a trifle condescendingly, as if such an obvious thought could never have occurred to me.
But Russian Village Priest instantly knew what I was talking about.