True ecumenism can be achieved with New Testament beliefs. When both Catholics and Protestants try to base our faith in traditions, we wind up in different silos (denominations, orders). But in the times we go back to believing The Word, God moves mightily in His church. Look at the First and Second Great Awakenings in the west. Look at the countless times Christian Ethiopia withstood attacks from Muslim armies. Yet look at how God's people declined when we get away from The Word and start believing what we feel like God wants or what someone tells us God wants.
The key to achieving productive ecumenism today is encouraging all of God's church to believing only in the Bible. We've learned enough over the years not to get caught up in bias interpretations and such. We know the exegetical beliefs that are worth drawing a line in the sand over.
The worldly people are mostly united in their hedonism and globalism they're forcing onto us. Nobody can talk them out of it. Can God's church be equally united in believing in The Bible? Or are we going to keep being talked out of it by going down denominational rabbit trails of tradition and denominational beliefs?
A big problem today is that many denominations, including Catholicism have decided that the Bible is not the inerrant word and is to be interpreted via the traditions of man.
This of course leads the the problem of what exactly can one believe if the written word is malleable and interpreted via culture and/or tradition, how does one search for absolute truth in such a model?