Zippy Catholic is a blogger who has written extensively about usury and its place in Catholic theology vs its occurrence in modern finance. He has dug quite a bit into the Magisterium along with the writings of St Thomas Aquinas on the subject.
Zippy’s (and Aquinas’) take is that there are essentially two kinds of loans: those that put the borrower on the hook for the return of the principal to the lender (called “mutuum” loans; aka “full recourse” loans in modern times) and those in which the lender’s recovery is limited to the property used as collateral (called “societas”; aka “non recourse” loans). Aquinas (and Zippy) draw this distinction between these types of loans; to them, charging profitable interest on a mutuum (full recourse) loan is
always usurious, never morally licit while charging interest on a societas (non recourse) loan is not necessarily so.
I won’t go into a full discussion here. Instead I’ll refer CAF readers to
Zippy’s excellent Usury FAQ here. The FAQ gives a few examples of what is or might be usurious lending to clarify the points Zippy is making. If that FAQ whets your appetite for more, you can check out
Zippy’s extensive archive of usury articles here. I have no idea if Zippy Catholic is a member of CAF (maybe under another name?), but I credit him for adding to my understanding of usury and I think his writings are an excellent resource for the discerning Catholic interested in viewing the modern finance environment in the context of Catholic teaching.