I see what you’re saying, but if you get an Episcopalian, a Baptist, and a Presbyterian together and start talking about doctrine, you find out that they are not all on the same page, and will disagree vehemently on very critical areas of Christian life and Church teaching on areas such as: Baptism for infants, church governance, eschatology, predestination, worship, etc… People like to think they are all generic Christian, but, in reality, they have a kaleidoscope of doctrines.
The reality is that there is only one truth, and Christ came to earth to teach us that truth. That truth has been here from the beginning, was here in the 500’s, the 800’s, the 1200’s, and today. It is something that we can know, now, with certainty because the same Church that was here in 800AD is still here teaching the same things on Baptism of infants, the Real Presence of Christ in the Eucharist, the sacrificial priesthood, Marian doctrines, Primacy of the successor to St. Peter in Church governance, etc…
These things are not incidentals - they are key to the whole truth Christ gave us. To minimize these things is to minimize the truth.
When people are talking about what divides us, they tend to approach it from a point of view that says, “What’s true for you isn’t necessarily true for me.” Instead of that, they should be saying, “Is this true?” The problem is that, if you are convinced of the truth, and it is not in line with your denominational creed, then you must either act on your new convictions or stifle them and stay where you are. Either way, there’s conflict on a personal level. So, it’s easier just to focus on what unites rather than what divides.
Sorry for the rant, but I think the whole “We’re all christians” is a smokescreen for avoiding real issues.