It always seemed like a strange thing to be specifically called out in either faiths holy texts about. How did religion get involved in what seems like such a nitty gritty matter of finances?
There are two elements here, for Christianity at least. One biblical and another deriving from Aristotle via Thomas Aquinas. The long and the short of the biblical portion is quite simple. The bible is not keen on usury. The old testament, which is accepted by Christians states explicitly in several places, "Thou shalt not lend upon interest to thy brother: interest of money, interest of victuals, interest of any thing that is lent upon interest," (Deuteronomy 23:20) and in other locations in Exodus, Leviticus, Ezekiel and so on. This prohibition is also present in the New testament, Luke 6:34 says "lend to them without expecting to get anything back" (i.e. you will get your money back and nothing extra) and Jesus drives the money lenders out of the temple.
That bit is relatively straight forward, fast forward about 1000 years and we get to the Scholastics. In the medieval period Greek philosophy began to filter back into the west, most particularly the philosophy and ethics of Aristotle. Aquinas, the Great Dane of scholasticism, tried to reconcile Aristotle's philosophy with Christian prohibitions on usury. Aquinas thought that money has a final cause (the thing for which something is made) that final cause is not the creation of new money, its final cause is to be a medium of exchange. The creation of money out of nothing is sinful. Money for him is what it is and holds its face value and does not change, yet usury diminishes the value of money by increasing supply and robs the person of the value of the money. Therefore, to use money in this way is a violation of natural law. He added that usury amounted to a form of double charging. When I buy something in a shop I do not need to pay an extra % fee to use the thing that I have bought, this is, according to Aquinas, exactly what usury requires. You can argue that his argument is a bit weak and ignores economic reality (inflation is a thing) but at the time his ideas were very influential. For instance medieval kings went to great effort to combat wage inflation (the black death when the price of labour skyrocketed) because to change prices of things was to disturb a natural law which was meant to be eternal.
Hi there -- there is quite a bit to be said on this topic, but you may also be interested in another current trending thread on usury that focuses on Christian-Jewish relations.