Vladimir Moss has also written that ecumenism in some form is ancient.
Although ecumenists can be found even in the early centuries of Christianity, ecumenism became a mass phenomenon only after the Second World War.
I am inclined to agree; in fact I do not think that there is anything "proto" about it. Rather I find upon reflection, that ecumenism has remained in essence the same.
The ecumenists say that our doctrinal differences do not matter and should not impede unity. Well, the emperor and his men invented monothelitism in order to create a false unity between Orthodoxy and monophysitism. Various uniates have for centuries tried to have false unity between Orthodoxy and papism. And before all of those, Arians did everything in their power to keep the Orthodox laity and clergy in their communion.
Just consider that for one second--we know Arius was a heretic, but he claimed that Orthodox Christology was the heresy. His followers lied, deceived, connived, and did everything imaginable to get people into communion with them both knowingly and unknowingly. By intentionally communing and uniting with the Orthodox--successfully or otherwise--they, like every other group who have ever hidden their heretical beliefs or created false unions, must necessarily have held an ecumenist view which allows unity with (what they consider) heretics. So did those who accepted the deal.
And before them, where do we find the same view? It is pagan in origin. The pagans have always and with very few exceptions believed that they can worship any and all gods at the same time and they will worship with those who also worship with contradictory religions. They allowed worship of Christ and still do.
The part which pagan persecutors had a problem with was the exclusivity of Christianity, and that is the same position which the ecumenist holds. The temptation to get along and have peace with the world has not changed. What has certainly changed, is that those with such an inclination--whether or not their true numbers have changed--have obtained the power to live out their fantasies in the absence of a strong force to depose them.
"They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us."