Dr. K. Scott Oliphint of Westminster Theological Seminary explicates something that I've been hinting at for a long time: that Thomism is the gateway drug to Roman Catholicism. He puts his finger right on the problem.
The reason this is so is because, according to Rome (and Thomas), human reason is a principium ("first principle") of faith, instead of Scripture.
Scroll to about 15:20 for the beginning of Oliphint's talk (or see also this link: https://youtu.be/cpSwqV2RnaY?t=15m20s).
Here are a couple more links about "Reformed Thomism":