Why ecumenism? Ask Pope John Paul II:
228. The justiﬁcation for ecumenism is not merely sociological, but evangelical, trinitarian, and baptismal. “It expresses the real albeit imperfect communion” already existing between “those who were reborn by Baptism” and the concrete testimony of fraternity.(Ut Unum Sint 96)
Pope Benedict weighs in also:
The magisterium insists on the trinitarian and baptismal character of ecumenical effort, where dialogue emerges as a spiritual and practical attitude, along a route of conversion and reconciliation. Only thus will the day arrive when “when we will be able to celebrate the Holy Eucharist together with all believers in Christ.”(Sacramentum Caritatis 56)
The Aparecida bishops endorse one path ahead:
A fruitful way to advance toward communion is to recover in our communities the meaning of the baptism commitment.
This makes sense to me. Christian disunity is a dark mystery. On one hand, it never should have happened. On the other, the Church is still partly a human institution, and our human flaws cloud and obscure Christ’s hoped-for unity. Christians share baptism, so perhaps that starting point is best for us to develop, reflect upon, and explore together.
For deeper examination, an English translation of the 2007 document from the Aparecida Conference.