From the Becker-Posner blog (this one from Richard Posner):
Medieval Christianity forbade the charging of interest on the ground that it was unnatural for money to increase (as by lending $100 at a 10 percent interest rate so that at the end of the year the $100 has grown to $110), because unlike pregnancy there was no mechanism by which an inanimate object such as money could reproduce itself. Behind this superstition lay undoubtedly a hostility to commercial society, which persists today in some quarters of the Muslim world; Islam forbids charging interest although substitutes are tolerated. The concern with lending has persisted into modernity even in Western societies. Usury laws, which set a ceiling on interest rates, and the Truth in Lending Act, which requires detailed disclosure of annualized interest rates in consumer loans, are examples of this concern.