This logical fallacy goes something like this:
If P then Q, Q therefore P
If the Bible isn’t clear then there will be a lot of different interpretations and denominations. There are a lot of different interpretations and denominations therefore the bible isn’t clear (necessitating the need for the magistarium).
The obvious problem with this fallacy is that there are certainly many other reasons why Q happens (i.e. illiteracy, pride, ulterior motives, etc) and therefore one cannot necessarily infer the antecedent (P) from the Consequent (Q)
For more information on this See (How Heresy Helps)