Even for all the years the various Christian churches were separated from each other, they certainly didn’t ignore each other. In fact, they needed each other—the evil foil to their own unspoiled righteousness. Protestants needed their caricatures of the pope and his evil ways, and the mindless Catholic minions who obeyed him unquestioningly; Catholics needed their stereotypes of modernist, rationalist, ultimately atheist Protestants who abandoned all truth. The result was noisy, dual-track monologues—neither side listening or having any good reason to listen. But it was also parasitic: my truth hinged on your absolute falsehood.
So it’s pretty obvious that long before churches could even consider merger, they had to get reacquainted and find out what the other was all about. A lot can happen in 450 years! Both sides had been through a lot and had changed in various ways. There was a growing recognition of the need to do some clean-up in their own houses. The place to start was conscientiously obeying the 8th Commandment: You shall not bear false witness against your neighbor.
Unitatis Redintegratio explains what this means:
“The term ‘ecumenical movement’ indicates the initiatives and activities planned and undertaken, according to the various needs of the Church and as opportunities offer, to promote Christian unity. These are: first, every effort to avoid expressions, judgments and actions which do not represent the condition of our separated brethren with truth and fairness and so make mutual relations with them more difficult; then, ‘dialogue’ between competent experts from different Churches and Communities.” (§4)
First, stop repeating your own caricatures, stereotypes, and outright lies; second, start paying attention to what the others are really saying about themselves. UR continues:
“At these meetings, which are organized in a religious spirit, each explains the teaching of his Communion in greater depth and brings out clearly its distinctive features. In such dialogue, everyone gains a truer knowledge and more just appreciation of the teaching and religious life of both Communions. In addition, the way is prepared for cooperation between them in the duties for the common good of humanity which are demanded by every Christian conscience; and, wherever this is allowed, there is prayer in common. Finally, all are led to examine their own faithfulness to Christ’s will for the Church and accordingly to undertake with vigor the task of renewal and reform.” (§4)
Ecumenical dialogue isn’t only good for individuals, but for the world—better understanding means cooperation is possible in addressing all the world’s enormous needs.