Catholicism promoted ecumenicallism in the first place ... early ‘70’s as I recall.
Ah, truth - but not the whole truth.
The unity of the body of Christ is first spiritual, unseen, and a fact, and the true body of Christ is one:
Eph. 4:3: “Being diligent to preserve (you cannot preserve that which does not exist) the unity of the Spirit in the bond of peace. THERE IS ONE BODY....”
External unity is found experimentally/experientially as we stand, believe, and live in the reality of this truth.
In doing so, we are then working “until we all attain to the unity of the faith, and of the knowledge of the Son of God....” Same chapter of Ephesians, verse 13. The unity of the faith is not here, not now, never will be. Not until we see His face.
The error is to try to maintain or preserve that which does not exist and that which we must be working toward: the unity of the faith. And to then try to work toward, or attain the unity of the Spirit - which is already a reality.
It will never work because we treat the two in opposite to what is commanded by scripture. That which already exists we are trying to achieve; that which does not exist, we are trying to preserve.
For a lenghthy, in-depth analysis of the problem (will be a too long read for most), see here:
Full disclosure & transparency require that I confess that this is one of my own writings.....
First of all the Church the Body of Christ is Holy, Righteous, Justified and Sanctified; absolutely perfect.
Religions are fractured, unholy, unrighteous and phony tools of the devil, 2 Cor 6:17 Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you.
Believing Christ died for your sins; is the only way to be saved. If there where any other thing that could send some one to hell other than sin, (and there is not) it would be all these world religions. There is a way that seems right to a man but they are the ways of death.
1 Corinthians 15:3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; 4 And that he was buried, and that he rose again the third day according to the scriptures: Believe this Gospel above ICor15:3,4 and you will be saved; Romans 3:23 For all have sinned, and come short of the glory of God;