Then, in the Middle Ages, Catholic theologians added nuance introducing a concept of time value of money - ie when you lend out $100 you also lose the ability to use that $100 for the time of the loan. The concept of a small interest rate was adopted.
Which is fine, except it opened the flood gates until we eventually got the high interest rates we have today.
What makes our rates usurious? That they are issued with the issuer knowing the principal will never be paid off.