I wrote first
"A low angelology is a doctrine [that] Tertullian insists upon even while he still actively criticizes Christians who teach other otherwise."
Then I looked at it, and rewrote it.
"Tertullian insists upon a low angelology even while he still actively criticizes Christians who teach other otherwise."
Not an overwhelming improvement, perhaps, but I think that it makes the sentence less clunky in style and content: a better flow. I might rewrite it again, mostly because I'm not satisfied with the verb "insists," and I think one should be very careful about adverbs ("actively"). In your rewrite of an essay, you should cut 50% of your adverbs, and probably just slightly less than that of your adjectives. (Later in your career you can pump those percentages up higher.) Remember: getting the idea "out" on to a piece of paper is not the same thing as communicating it clearly and succinctly.