One of my confreres has suggested that I write a book about Baianism, the heresy named for the 16th-century Louvain theologian Michael de Bay (a.k.a., Baius). Nobody, as far as I know, has written about Baianism since the 1800s, and that was in French and German (or Latin).
Baius was a zealous Catholic reformer steeped in the ressourcement of Renaissance classicism--in his case, more a return to patristic sources, of course, than to pagan letters. He was a man without sympathy for medieval theology or philosophy. His proposal, which ought to sound trite to everyone by now but doesn't, was to recover the pure vigor of patristic Christianity by bypassing scholastic thought and grasping Biblical and patristic Christianity with the tools of critical scholarship. In philosophy he was apparently a less-than-reflective nominalist.
The errors that were condemned as Baianism stand in an uncertain and uneven relationship to what Baius actually wrote and taught. (This makes writing about Baianism easier, since it lets me put aside the quest for the historical Baius.) It seems to me that the 76 propositions rejected by Pope St Pius V in Ex omnibus afflictionibus (1567) can be boiled down to seven errors.
1. Baianism treats moral good and evil exclusively in terms of obedience to the will of God. There's nothing morally intelligible for us to consider in human acts themselves, such that we could conclude that an act is intrinsically or objectively good or evil. The appeal, for Baianists and others, is that this leaves God absolutely free to command whatever He wills; it also rids us of moral theology.
2. Baianism identifies the voluntary with whatever we will without external compulsion. Internal compulsion is not only A-OK, it is inevitable: everyone is a slave, either of charity or concupiscence. If God moves anyone to act in charity, He imposes the necessity of charitable action on the creature. Baius could have used St Thomas's clarification here, to distinguish between the unfailing effectiveness of God's will and it's imposition of necessity.
3. (I'll have to take this up later...)