Christian Smith on the necessary distinction between dogma, doctrine and opinion:

“Some Christian beliefs are nonnegotiable for any believer—such as the dogmas of the Trinity and Nicene Christology. Other beliefs are those to which groups of Christians adhere with firm conviction but also disagree over with other kinds of Christians—such as Calvinist or Wesleyan systems of theology. Still others are beliefs that some Christians hold, sometimes with strong feelings, but that are far from being central, sure, and most important in the larger scheme of Christian belief and life. Examples of the latter include a preference for baptism by immersion rather than sprinkling, the commitment to homeschooling children versus sending them to Christian or public school, and so on. The most central, sure, and important of these beliefs we may call “dogmas.” Those occupying the middle range of centrality, sureness, and importance are in this scheme called “doctrines.” Those which are the least of these let us call ‘opinions.’”

Read more in his book The Bible Made Impossible.