Using tags as a folksonomy as in Shog9's answer, I can see what I think are valid use cases for the tag. We might easily have an expert on different doctrines of justification, and to them the tag combination justification doctrine would help searching for interesting questions.
In the general case, tagging by the name of the doctrine won't work, as in all cases the different doctrines don't really have names but are rather characterized by the name of the church they are used in. We can't really have lutheran-doctrine-of-justification.
Now, I will admit that my own use of the doctrine tag has probably been bad, as has my use of tags in general. I will try to correct my ways.