Ecumenism, to be pursued, must consider the three realms of what constitutes faith. Would such pursuit be fruitful? Ecumenism necessitates a joining together, a subsuming of one by the other. Logically, it cannot happen.
(1) Governance. Two completely different systems prevail. Both claim apostolic succession. One is historical from the first century after Christ; the other is restored and historical only from the 19th century. Peter was appointed to be the rock upon which the Church would be built, by Christ himself. One system asserts the successors of Peter in his see alone were Linus, Cletus, Clement, Sixtus, and over a hundred and fifty others. The other system asserts succession from a visionary appearance of Peter, James, and John to ordain a totally new line (by the way, LDS friends, what is the exact date of this event?–on what date was the “Melchisedek priesthood” actually restored?–be specific, please). We can appreciate apples and oranges, but is combining them even possible? Ecumenism would fail here.
(2) Sacrament. Two completely different systems prevail. Let us consider, for the moment, the roots of the word “sacrament.” It comes from the French, which in turn comes from the Latin. Many words in the early Church had to be taken from common usage and given new meanings (much as we would invent words in our own time, such as “bit” and “byte” for computer data, “internet” for this marvel of communication, “astronaut” for someone who flew to the moon). When Christ instituted the sacraments, he introduced something new to human history. “Sacrament” acquired a working definition as “sacred means,” these new, saving, mystical avenues through which God gives grace and humans accept it by outward and visible signs. For Catholics, there are seven of these sacred means. For LDS, there is but one meaning for the word, the bread and water commemoration of the Lord’s Supper. Instead, LDS have ordinances, which are said in some cases to date back to Adam. They are not the same thing.
(3) Creed. There are significant insurmountable issues here separating the two systems, starting from the very words to be used in framing the discussion. Words like: apostasy, scripture, Sacred Tradition, redemption, atonement, grace, conceived by the Holy Spirit, forgiveness of sins, original sin, salvation, exaltation, correct translation, and so forth. Even using the Apostles’ Creed, rather than the Athanasian Creed (and all its incomprehensibility as so often quoted in LDS literature as a “clear statement”), as the baseline for discussion of what Catholic Christians believe, there is no comparison with the LDS Articles of Faith.
We can be friends. We cannot be united. Enjoy the conversation.