In medieval finance, a gage could come in two forms: a mort-gage or a vif-gage. When the owner of property needed liquid resources, they could exchange their property, as a surety

Surety

A surety or guarantee, in finance, is a promise by one party to assume responsibility for the debt obligation of a borrower if that borrower defaults...

, for 'cash.' Since the gage was typically a piece of property which generated revenue (i.e. a farm which produced crops, a mill which processed food, a pasture that provided milk or wool, etc.), the creditor received the benefits of the land. Under the terms of a vifgage, these benefits reduced the amount the borrower owed, while under a mortgage they did not. This meant that if the property was prosperous enough, or the loan small enough, a property in vifgage could pay off the debt itself. On the other hand, with a mortgage, the benefits of the property constituted interest on the loan, which made it a form of usury

Usury

Usury Originally, when the charging of interest was still banned by Christian churches, usury simply meant the charging of interest at any rate . In countries where the charging of interest became acceptable, the term came to be used for interest above the rate allowed by law...

. This meant that mortgages were seen as immoral/illegal among Catholic theologians.