There are three principle ways in which one can be involved in the ecumenism, the cause of Christian Unity.
The first is Prayer. On the night before he died, Christ prayed for his disciples, “May they all be one…so that the world may believe that you sent me.” (Jn 17:21) So first, we must join our prayer to that of Christ. Since all Christians share a common baptism, we can pray for unity as individuals, in groups, and even in certain liturgical settings such as the Liturgy of the Hours. Sadly, because of the very real differences in faith and understanding, we cannot yet share the Eucharist with other Christians.
The second way to be involved in ecumenism is to work with other Christians in projects or programs which address areas of common concern. What we can do together, we should do together. In my hometown, all the churches get together to provide social services for needy people. We also have a common food pantry. Thanksgiving is an especially good time when we pool our efforts to collect the necessary items for hundreds of Thanksgiving baskets. Our parish was always in charge of the potatoes. Go figure.
Finally, the third way to be involved is to participate in dialogue with other Christians. The first step in dialogue involves getting to know the other as they really are, not just as we suppose them to be. Find out what you hold in common first. Usually when people enter into an intentional dialogue they find that they have much more in common than divides them. Once trust and respect is established, then real work can begin on the things that are divisive.
Ecumenism is about building relationships; relationships with Christ and with one another. As we get closer to Christ, we get closer to one another. Eventually, perhaps many decades or centuries from now, we may indeed all “be one…so that the world might believe.”