Ecumenism cannot truly be understood apart from its Masonic - which is to say, Kabbalistic - origin. Albert Pike wrote, on page 590 of Morals & Dogma:

“The Kabbalistic doctrine was long the religion of the Sage and the Savant; because, like Freemasonry, it incessantly tends toward spiritual perfection, and the fusion of the creeds and Nationalities of Mankind.”

At the heart of this doctrine is the notion that all human souls are emanations of God and, as such, fundamentally and ontologically divine. Within that paradigm, so their argument goes, various religious creeds and confessions all obscure the gnostic “truth” of our shared divine unity. 

It is not difficult to understand why they put so much effort into blurring dogmatic boundaries and doing everything in their power to get the Body of Christ to compromise with heterodox (and outright heretical) confessions.