Answering Jesus' prayer for the unity of his disciples is an immense challenge as previous posts here have indicated. But ever the optimist I suggest we do not descend into ecumenical gloom and doom.
One specific reason for not being gloomy about future prospects for one global communion of all Christians is the mighty power of the Holy Spirit. We should never act or think as though the Spirit is constrained by our own lack of ability to foresee better days to come. We could also contemplate all that we can see that the Spirit is achieving among us in our day.
Here I want to begin a list of the things which actually unite us. You could help by adding to the list in the comments. The list, I suggest, offers both reason for ecumenical thanksgiving as well as hope for the future.
In no particular order of significance ...
1. Christians accept the Apostles' Creed.
2. Only a few words of the Nicene Creed divide Christians who otherwise accept all the other words of this creed.
3. Without specific heads of churches agreement forcing the matter, from the ground up many churches are united in following a common lectionary. In itself this is a recognition that Scripture (noting 4 below) itself is a continuing matter of global commonality for Christians.
4. While acknowledging differences concerning the canon of the scriptures between Easter and Western Christianity, and within Western Christianity, all Christians are united on the canon of New Testament Scriptures.
5. Trinitarian baptism is practised by the vast majority of churches, and, with exceptions such as the Salvation Army, baptism remains the common initiation rite of those churches.
Can you add to this list? Do you disagree with one or more items on this list?