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3) that explicit belief in the Trinity is only "necessitate praecepti" and that only implicit belief in the Trinity is required "necessitate medii." He calls this third opinion (which he lists as the second) "also probable enough" and he quotes a number of eminent theologians who hold this opinion. Here is the passage in his works:
https://books.google.com/books?id=NR48AAAAMAAJ&pg=PA296

Who has a problem with the above quote?

Who has a problem with the "above" methodology?

Whoever wants to answer the question can.

 "Right?"

I'm basing it on what Alphonsus taught above.  

And the mistaken opinion on St. Alphonsus was due to a lack of deeper understanding regarding the Ordinary Universal Magisterium ... which wouldn't be defined clearly until Vatican I.  Pursuant to the Vatican I definition, St. Alphonsus would no doubt have revised his opinion.  He simply saw a number of prominent theologians (big-named Jesuits) proposing this opinion and he called it less probable for that reason, because a minority of theologians held it.

But just because something remains uncondemned with explicit condemnation doesn't mean it's not heresy or error.  Again, Lutheranism, Pelagianism, Arianism, Nestorianism ... all these flourished for a significant length of time before they were explicitly condemned by the Church -- that did not make Arianism a "less probable" opinion, simply because it remained uncondemned; Arianism was a heresy from the inception.  It's just that it took people a little while to wake up to it and explicitly condemn it.