It seems there are three ways to approach ecumenism: the nice approach, the eschatological approach, and the co-belligerency approach.
1. The nice approach
The first is to look at the differences between ourselves and dismiss them, even the big ones. Don’t believe in the virgin birth or the resurrection? No problem; pull up a chair! I don’t believe this approach honors God or the testimony of our faith, as it essentially says such things are unimportant.
2. The eschatological approach
The second is to stand together now on the assumption that our differences will over time (maybe even centuries or more) grow fewer or less pronounced as we all grow more in the image of Christ. Even if you’re coming from opposite sides of the globe, the closer you get to true North, the closer you get to each other. I think this is appealing in that it assumes the best of our neighbor and that grace will have its effect, while not insisting that we cheapen our testimony by turning blind eyes to serious disagreements in the present.
Call this the eschatological approach, as it will only find its fulfillment in the coming kingdom of which we are now partially part. This obviously works best with those with whom you already share some significant common ground.
3. The co-belligerency approach
The third option is perhaps best called the co-belligerency approach. Baptists and Presbyterians and Catholics and Orthodox and Pentecostals have many areas of disagreement. But they are in large agreement on basic Christian morality and ethics, and also in the Triune God of the Bible and Creeds (at least in their content if not always their confession). Such Christians can stand in solidarity wherever they find such common ground.
From this angle, it’s actually quite similar to the second approach listed above, but it could be expanded beyond that.For the sake of accomplishing certain ends (perhaps, e.g., charitable or political), alignment could be made with Mormons, Muslims, Jews, and others who disagree with Christian doctrine but who find agreement in morals, social norms, etc.
While I think the first option is a dead end, I think there is a lot of value and merit in thinking about approaches two and three.