I believe the prohibition of interest has been a common feature of the moral rules of several civilizations. Interest was condemned by Aristotle and generally by the ethical systems of the ancient world. It was treated as a sin by the medieval Catholic Church and subject to bans and punishments. And Islam also forbids interest today, a rule that Muslims, unlike modern Christians, still regard as binding, which has driven the emergence of "Islamic banking."
Contemporary economists, by contrast, habitually regard both legal prohibitions and moral strictures against interest as pure superstition. They regard interest, on the contrary, as both necessary to the functioning of the economy, and as morally innocuous. All the prohibitions of interest by philosophers and ecclesiastics for so many centuries they regard as based on the simple fallacy that regards it as "unfair" to get more money tomorrow for money lent today because of the inequality of the sums of money involved. But of course, money-today is not the same commodity as money-tomorrow: for one thing, prices may have changed; but even if they haven't, there is a set of opportunities between today and tomorrow to which money-today gives you access and money-tomorrow doesn't. For people to exchange money-today for money-tomorrow is perfectly sensible and rational on both sides, like trading haircuts for nails or bread for cars or any other ordinary, voluntary, mutually beneficial exchange, and there's no reason that the price should be $1:$1.
Who is right? Mostly the economists. Interest is certainly necessary; it is often innocuous and useful; prohibiting loan interest is a very bad idea; and the motive for past prohibitions has usually been simple fallacies combined with the aristocratic class prejudice and philosopherly impracticality. However, there is one valid argument on the other side. Laboratory experiments have shown that people often exhibit hyperbolic discounting behavior, a fancy term meaning that they place a higher relative value on present enjoyment than is consistent with their desires when they are taking the long view. It seems to be human nature to be somewhat impulsive and to behave in ways prejudicial to one's long-run well-being, to the extent that people sometimes invest in costly self-control mechanisms. (Example: If I remember right, it has been shown statistically that people buy gym memberships even though it would be cheaper to pay per visit; the argument is that they foresee this, but are trying to bind themselves to exercise regimens by lowering the marginal cost of visiting the gym.)
If (some) people are impulsive, other people can exploit this weakness, giving them money now for prices in future money that their long-run selves would not agree to. How to do welfare analysis when people have time-inconsistent preferences presents difficulties, but plausible intertemporal utility functions would support the folk economics idea that loan sharks harm their customers. It might be useful to define usury as lending at interest that exploits people's impulsive short-sightedness, and interest as lending at interest to rational borrowers who have good reasons to place especially high value on money-today. Of course, an external observer couldn't always tell, and you might never be completely certain, whether any given transaction was a case of usury or interest. Even the lender and the borrower might not know. Indeed they might know least of all: they would have the best information, but maybe also incentives for self-deceit.
I think the Christian attitude should favor interest, thus defined, and oppose usury. It should be regarded as a sin to lend money at interest if the lender has strong reasons to think that the borrower is being impulsively short-sighted and will regret the loan later. It might also be good for the law to prohibit usury. This is paternalistic, but perhaps not coercively so, The law could (maybe it does, I'm not sure) simply say, "We define such-and-such contracts as usurious and therefore invalid, and we will not enforce them. If a usurious lender wants to simply trust his borrowers to repay, we won't interfere, other than to make it clear to borrowers that they are entirely at liberty to walk away from such illegal debts; of course, a lender who tries to recover his money by any but purely social and moral pressure will be treated as any other extortionist."
But of course, the problem is how to prohibit usury without prohibiting interest. That can't be done perfectly. The moral usury/interest distinction will not correlate perfectly with any particular interest rate or form of contract. A loan at 0% interest might be motivated by instant gratification, while a prudent entrepreneur may occasionally have sound business reasons to borrow even at very high interest. Some entrepreneurial ventures might be recklessly self-indulgent, while other loans purely for present consumption (think of a payday loan to provide a wedding reception for one's daughter) might be quite consistent with making the best use of one's resources in the long run. The ideal might be a combination of law and moral suasion. Usury laws should prohibit the most abusive forms of lending, even at the expense of occasionally curtailing entrepreneurship; but they should allow a wide range of lending practices and rates, even though this will leave the door open to a good deal of substantive usury and exploitation. Meanwhile, churches and civil society organizations should use exhortation and stigma to shape norms so that moneyed persons will use moral discernment to try to lend only in a manner that benefits their borrowers.
A system of "morally sound" financial practices as described above would likely have prevented much of the crisis. Of course, we have some of it already: usury laws are on the books. Yet all in all our society is the opposite of the medieval Catholics: the state not only does not prohibit lending at interest, but subsidizes it. Student loans are one instance of this, and perhaps the policy of putting young people in so much debt gets less criticism than it deserves. But the most egregious example is the mortgage-interest tax deduction, which creates strong incentives for much of the population to become deeply indebted. It's clear now, in the wage of the largely mortgage-driven crisis of 2008, that much of the home mortgage industry was usurious in the sense described above, yet this industry was, and still is, being subsidized by the tax code. Surely this is perverse and should be abolished. We shouldn't go back to medieval prohibitions of usury, but perhaps something in between?