In the Old Testament, the law forbids the Israelites to exact usury from their fellow Israelites (i.e. make loans with interest).
The Medieval Church applied this to all Christians, making the occupation of a banker a pretty poor choice for a career. Oddly, the Jews (the ones the Law was actually given to) had no problem with it...lol.
And while usury was forbidden, some groups like the Knights Templar found loopholes. Instead of charging interest, they would charge "rent payments" people who deposited money with them. Pretty clever I guess.