When we assert one version of a story we eclipse others which, if allowed to be heard, might shift our views into another political position. In academia, it is often easy to shift perspectives when in the classroom, because of the distance we have from the material. No one will check up on us later to make sure all our actions conform to what we've said in class.
But merely saying that we agree with an opinion does not necessarily mean that we have taken up an ideology. In order to do that, we must perpetuate the ideology in some way, by taking up the language of its discourse or somehow making that discourse part of our self-narratives -- the stories in our minds that tell us who we are, what we stand for, and why.
For example, what happens when you hear the President of the United States say on television, "My fellow Americans...." ? Do you identify with this group? Do you identify with the ways in which he defines you? Does his sense of group spirit persuade you to believe that he speaks on your behalf? If you accept this "hailing" (as John Fiske explains in "British Cultural Studies and Television," Channels of Discourse, Chapel Hill: University of North Carolina Press, 1987, 258-59) and allow it to become a part of your image of yourself and your place in the world, then you are taking up (whole or in part) an ideology.
N.B. I have borrowed the "My fellow Americans" example from Ashley Smith, a Brown graduate student, who taught a course called The Politics of Representation in 1989.