Steve Ray has a list of 35 loaded Questions for "Bible Christians" (quotation marks his)(link to the whole list). This is number 19/35. I'm trying to provide the answers in a common format, for easy reference.
19) Protestants usually claim that they all agree “on the important things.” Who is able to decide authoritatively what is important in the Christian faith and what is not?
Everyone, no one, and Scripture.
1) Which answer to use depends, of course, on the sense of "decide authoritatively."
2) Scripture is the authoritative standard for such a question.
3) Everyone who is a believer is competent to read and understand Scripture and to seek out an answer to the question of which are the important things.
4) No one is infallible.