This is one of the many reasons why the Church always forbade the type of ecumenical practice propagated since Vatican II, because it places the One True Religion established by Our Lord on the same base level as man-made creeds.
In fact, 2000 years of Catholic Teaching condemning ecumenism is summarized in Pope Pius XI’s 1928 encyclical, Mortalium Animos, which was a prophetic, wholesale condemnation of Vatican II‘s ecumenism. It was here that Pope Pius warned:
“… it is clear that the Apostolic See can by no means take part in these assemblies, or it is in any way lawful for Catholics to give such [ecumenical] enterprise their encouragement and support. For if they did so, they would be giving countenance to a false Christianity, quite alien to the one Church of Christ. “
Likewise, the Scriptural condemnation of religious affiliation with non-Catholics is masterfully demonstrated by Bishop George Hay, especially in his book The Sincere Christian, under the heading, “On Communicating In Religion with Those who are Separated from the Church of Christ. “
From the book: The Revelations of the Holy Face of Jesus by John Vennari
Reality Meets Faith 