The question at hand is: how is ecumenism even possible? If we disagreed enough in the past to split into different churches, how can we claim sudden agreement now without totally selling out everything we believe in?
I already covered the possibility of misunderstanding. Now I’ll turn to the matter of differing traditions within each church.
Except for the tiniest little sects—and probably even there—all churches harbor a range of opinions and convictions. This is not automatically a bad thing, and often a strength; one single person can’t recognize or understand every last issue in theology or ethics or cultural engagement. We’re meant to be many members of one body (I Corinthians 12).
And this means that there will various developments and streams of tradition even inside of one single church. Think for instance of all the different religious orders in the Catholic Church: they share the same faith but they have different areas of emphasis.
Of course, it’s only natural that some will be stronger in one area than another. Or that a tradition needs time to explore the consequences of its own ideas… which may eventually be judged: nice try, but wrong. Churches need a certain amount of freedom to make mistakes along the way, because they clarify the truth that much better.
Now one of Luther’s major criticisms of the church of his day is that it taught works-righteousness. Lutherans ever after have assumed that justification by works is THE Catholic position, end of discussion. But this is not correct.
Yes, there were some Catholics who taught works-righteousness. But not all Catholics taught it. The ecumenical research of the past century has realized that there were (at least) two major schools of thought about justification in the Catholic church leading up to the Reformation.
One school, following Gabriel Biel, thought that the “grace” of justification was given to a Christian only after the Christian, entirely by her own natural powers, produced a true and genuine love of God—this merit “earned” God’s grace. Luther rightly recognized this theory as Pelagianism in disguise, and that’s why he condemned it.
But other Catholics, following Augustine and Thomas Aquinas, taught justification by faith—and they insisted that faith is a divine work, not a natural human power. During the Council of Trent, the view of justification that won the day was actually more like Thomas than it was like Biel—and therefore more like Luther than like some Catholics! But for the next five hundred years, practically speaking, most of the Catholic church followed Biel instead of Thomas.
So what you see in the Joint Declaration on the Doctrine of Justification signed by Lutherans and Catholics in 1999, when it says in §3.15 that we are justified “not because of any merit on our part,” is that the Catholic Church finally decided it had to follow one of its traditions and reject another.