The argument from authority is a strange thing. No less an authority than St. Thomas teaches:
although the argument from authority based on human reason is the weakest, yet the argument from authority based on divine revelation is the strongest.
When you have an argument that is either the weakest or the strongest depending on its basis, you'll want to keep an eye on the basis.
So, for example, if someone rejects clear and explicit Church teaching with a counterargument that begins, "Many theologians say...," remember: Cherchez le basis.
Fortunately, in this example, the basis is easy to discern. The authority based on divine revelation is Holy Scripture and Sacred Tradition as interpreted by the Church's Magisterium. This, of course, is the authority with which the Church speaks, so the argument, "The Church says...," is the strongest form of proof.
At the same time, this is not the authority with which "many theologians" speak, so the argument, "Many theologians say...," is the weakest form of proof. The strongest proof contradicts the weakest proof. Which one proves its claim is easy to see.
Suppose, though, that there is no clear and explicit Church teaching the "many theologians say" argument contradicts. The rule is the same, and the conclusion -- this is the weakest form of argument -- is also the same.
Those who offer such arguments may well be faced with counterarguments like, "Yeah? Well, many small businessmen say...," or, "Many grandmothers say..."
Which, if any, argument should win the day is not something that can be determined ahead of time. Theologians do not necessarily speak with more authority than grandmothers (I know I've never heard a theologian speak with more authority than my grandmother did). Schooling, academic credentials, and lists of publications do not settle the question, "But is it true?," any more than the absence of these things does. (I'm no more fond of the "she's a little old lady who goes to daily Mass, so she must be wise" myth than of the "he's a tenured professor of theology at a university in the Catholic Tradition, so he must be sound" myth.)
If it's a question of theology as an academic subject -- how did Jansenism develop, say, or what are the open questions on the doctrine of predestination -- then a theologian may speak with more authority than a small businessman (though of course a particular theologian may not be particularly capable). But if it's a question of what God has revealed to mankind, a question of what St. Thomas called "sacred doctrine," then we look to the gifts of knowledge and wisdom the Holy Spirit gives, not to the job title a university gives, to judge the authority with which a person speaks.