There are probably some things all people who call themselves Christians believe, but we don't call them doctrines because the word doctrine tends to include the idea of a distinctive belief in the term itself.
Probably all Christians believe Jesus was the messiah for example. And all Christians believe God exists. Christians believe humans are sinful in some sense.
Unitarians are perhaps the outliers who would test even some of these basic beliefs. But then we could ask what it mean to be a Christian anyway and do liberal Unitarians qualify. Does anyone who calls themselves a Christian get to be defined as Christian, regardless if they deny sin, Jesus and even God himself? If so it all becomes a bit meaningless trying to define doctrines or groups or anything.