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These are good answers (in the posts above).
One more thing that should be recognized is that a technique of Modernism is to take traditional Catholic words and use them repeatedly in a new way so that over time the meaning of the words are changed into meaning useful for the spread of Modernism.
Ecuмenism is actually a new word. Before Vat.II, the prior 20 councils were called Ecuмenical Councils, so ecuмenical was a traditional Catholic word. What made a council of the Church ecuмenical was when all the bishops of the world traveled to one place to meet, in response to being called to come there.
You can say a Council is Ecuмenical when the Pope calls all the bishops, but it doesn't have to be the Pope who calls all the bishops, since several of the first Ecuмenical Councils of the Church were called not by the Pope but by the Emperor, and at some of those, the Pope never even showed up. But they still became recognized as Ecuмenical Councils because later, the Pope recognized them as such.
Therefore, to be ecuмenical is to be representative of the Church all over the world. But what then is "ecuмenism?" If it is true representation of the one true Church, it is Catholicism. So why would we need another word, ecuмenism?
The Modernists introduced ecuмenism because they wanted a word that sounds better than indifferentism, especially since the latter had already been condemned by many good Popes in the past. Consequently, ecuмenism is a made-up word that would be unnecessary unless it were for the purpose of deception and subterfuge, to destroy the Church.
Modernists use "ecuмenism" in the same way that our language would use "indifferentism" except with one difference. The proper use of "indifferentism" would make clear the fact that the Church does not condone it, and it is against the will of God, and it is evil. But by using "ecuмenism" in the same place it can be made to seem that the Church is suddenly okay with it.
It's like a magic trick where something suddenly appears to be altogether different from what it had been before, and the gullible audience becomes acclimated to the new reality before their eyes, even if there may still be some lingering doubt.
We are wont to say false ecuмenism to impart clarity to our words, but you will never see the Modernists use "false" in front of ecuмenism. I suppose that means that you can more readily tell when the words are trustworthy on that basis. Modernists abhor clarity and truth.
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