Not a trick question, here—when I say “All Protestants” I’m referring to all Protestants currectly versed in their particular denomination.
When I say “Agree Upon”, though, I mean “accept a common practical definition of and agree in all significant ways as to its implementation.”
You might say, for example, the Trinity, but if T.D. Jakes or some significant denomination under the Protestant umbrella does not hold to Father, Son, and Holy Spirit in one person this would fail the test.
Significance will be an unavoidable question—with so many Protestant denominations one must note with charity that there are many tiny offshoots which do not rise to significant influence and should not count against the doctrinal unity which does exist.