When people discuss the Catholic teaching on abortion or contraception, someone usually says something like, "Well, these teachings are subject to change. I mean, look at how the Church has changed her teaching on usury and slavery over the course of the centuries!"
Has the Church really changed her teachings?
The quick answer is: no.
The longer answer takes a bit of thought.
Usury is the charging of interest on a loan. In the early Church, this was forbidden in every instance. It is easy to see why.
In a subsistence-level society, where stored food can make or break an entire community's chance at survival, money is a consumable like water or food. In such a society, I can give you my bread today, but I risk having none tomorrow. Similarly, when you pay me back by giving me one of your loaves tomorrow, you can't afford to give me more than you received. If I demanded more, you might well starve. There's little enough to go around as it is.
There is no such thing as investment income. Money is good only because it can be stored on a shelf far longer than that salted venison or the dried fish we caught in the spring. All of that will eventually have to be consumed or it will go bad. But gold, silver, anything that has a longer shelf-life than the consumables it can be traded for, has value.
That's the upside to money. The downside is, you can't eat it.
I can't give you bread, and then charge you extra if you choose to eat the bread. Bread is made to be consumed. In a subsistence-level economy, money is also made to be consumed. I accept your gold for my bread ONLY because I expect to be able to spend that gold on bread for myself tomorrow. For us, money is like food, fire, clothes or any other barter material - it is consumed in the transaction. If I took your money in exchange for my food, I may very well not have enough food of my own. I will need to swiftly trade your money for more food.
If I can't "eat up" the gold you give me, that is, if I can't spend it, I'm not going to give you my bread no matter how much gold you have. I would starve if I did. The gold you give me today I will spend tomorrow to get bread for myself. Gold, bread, wine, it's all the same thing really. All of it will be stored, eaten, used up. That's what it's made for.
So, I can't expect you to repay me the loaf of bread AND expect you to pay me another half loaf for the privilege of eating the bread I gave you. Bound up with the very nature of bread is the expectation that it will be eaten. Asking for more bread than I gave you is charging for something that doesn't exist. It's asking you to pay for a right that is already given to you when you got the bread to begin with - with the bread, you also got the right to EAT that bread.
Catholics aren't allowed to charge for things that don't really exist. That's usury. Charging interest on a loan was, for the subsistence-level society, identical to asking for a loaf and a half back on every loaf given out. That's just evil. I'm killing people when I demand it.
Unlike cows, sheep, or grain, gold doesn't breed, so I can't ask for more than I gave, since the very act of asking would impoverish you.
But times changed. As societies grew beyond subsistence level, they found they had more barter stuff, that is, more gold, wheat, spice, cloth, then they really needed. What to do with the extra?
Some smart person somewhere realized that he could use his gold to barter with people at a far distance. He could invest the gold in a caravan or a ship. If the caravan actually returned with what he had bought, he would have a lot of barter material. His ship would have come in.
On the other hand, if he invested all his money and it was waylaid by weather or bandits, he might very well starve. There was a risk in making such an investment. A LOT of risk. It was smarter to share that risk with others. The only way to share the risk in a fair way was to share the wealth that would come if you beat the odds.
This is the beginning of investment banking. Once I have transport and excess goods, I find that gold is no longer a consumable that disappears because I have to eat. Now gold - or any money that may be used in its place - is a way to assess risk and investment, it has changed to become a marker in a much larger game.
If I am investing my money, putting it at risk in the hopes of greater returns for all involved, then I have a perfect right to share that risk out amongst all the investors involved. The investors likewise have a right to demand a share in the reward - a dollar and a half for the dollar invested. This is the charging of interest on a loan, but it is no longer usury, because the risk is very real, and the return on the investment is also very real.
Now you see the problem.
The Church has never changed her teaching on usury. It's still a sin. You can't sell something that doesn't exist. The teaching didn't change, but the cultural definitions of "money" and "interest" did. The definition of money is dramatically different now than it was in the first millennium of the Church's existence. Now money is an electronic cypher in a computer somewhere, a set of bits flipping back and forth, the swap of electrons on a silicon wafer.
We have changed the meaning of money.
In a similar way, God can change the meaning of who we are.
Prior to marriage, every consensual heterosexual act is fornication. But when a person gets married, he can no longer commit fornication, he can only commit adultery. Furthermore, while he can still sin sexually, he can now engage in consensual sexual acts that are not sin at all, but are actually a means to holiness.
The sacrament has changed him. Since the definition of who he is has changed, so the definition of how he can sin and how he can be virtuous has also changed.
In exactly the same way, the definitions of money and interest changed so that what used to be a sin can be transformed into an acceptable act. What I could not do in a subsistence-level barter system, changes into something I most assuredly CAN do in an affluent society in order to assess risk.
So, we can still commit the sin of usury, that is, we can still charge for something that doesn't exist or charge extra for something that doesn't deserve an extra charge, but we don't necessarily commit that sin by charging interest because the secular definitions of the traditional terms have changed.
Church teaching hasn't changed.
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