The Church’s stance against “usury” changed because of a change in social context and a recognition that such a thing as a ‘secular community’ exists. Simply put, the Church opposed usury for the same reason that ancient Judaism forbade usury when lending money to fellow Jews. A Jew was, however, perfectly free to charge interest to a Gentile who was indebted to him. But, he was forbidden to treat a fellow Jew this way, so as to promote a spirit of charitywithin the tribe, and not to create any resentment between fellow Jews. And the early Christian Church naturally maintained this Jewish sensibility (part of its own Jewish heritage), and so forbade usury between Christians. And, since the New Christian Covenant was recognized to follow a higher degree of righteousness than mere Judaism, it not only refrained from charging interest to fellow Christians, but also even to non-Christians –the idea being that all mankind is potentially part of the New Covenant. And, indeed, when the Roman Empire became officially Christian, you suddenly had an actual civilization which presumed that every member of the society was part of the New Covenant, and so to be treated with deference as a fellow Christian. This was the cultural situation in which the Church formally condemned usury during the Middle Ages –a time when only Jewish (or Muslim) bankers were free to (openly) charge interest on debts …although certain Christian bankers (e.g. the Knights Templar) got away with it too, by calling it a “donation” rather than “interest” on a loan. 🙂
Yet, starting with the Renaissance, which saw the rise of the great Christian banking families (like the Medici), the Church began to realize that there is a distinction between Christian (religious) society (i.e., inter-personal society) and secular society. Charging interest as a matter of pure business (within a secular society –aka, the “business world”) is not the same thing as usury …or charging interest to someone with whom you have a covenant bond. Ergo, charging interest in “the business world” is not a violation of inter-personal charity, as would be the case with your own brother (whether by blood or in Christ) or with someone else who you supposedly love and are obligated (by covenant) to help without selfish profit. To do such a thing (i.e., to charge me interest if I ask you to loan me $20 until the end of the month) WOULD (arguably) be the sin of usury. …Because you would be taking advantage of me (your friend and brother in Christ), rather than helping me out of sincere charity and love, as our relationship should require you to do. See the difference?
So, the development of capitalism and modern finance (a shift in societal context) is what forced the Church to change it’s position. But, truth be told, the Church’s position hasn’t really changed (as I just illustrated in my example above). It has merely re-applied its concern to the appropriate and pertinent context, which is inter-personal relationships, rather than the operations of the business world.