If members of a group routinely ascribe ideological positions to the group as a whole (as atheists frequently do with Materialism and Rationalism) like the OP did and never, or VERY rarely, get push back from members of the group, I would consider that a belief held by the group. Is that weird?

Like I said, I am an atheist. And because of that, look how easily (unconsciously?) he ascribed to me and all other atheists the ideological positions of materialism and rationalism.
That's what I mean by a doctrine

Materialism (by which I mean the belief that only the physical world exists)
And
Rationalism (the belief that reason is the universally best (or only) way to give true accounts of things in the world)

I disagree with this

While technically true, as an atheist that does not share many doctrines of Athiesm, I can tell you that, practically, Atheism does have doctrines. This post, for example, presupposes both metaphysical (materialist) and epistimological (rationalism) positions.