I had actually heard that, they essentially acted as a "pilgrimage insurance and bodyguard," at least in theory. Their growing wealth may have had a lot to do with why they were excommunicated, tortured, and killed.
But here's what I was talking about " And Jesus went into the temple of God, and cast out all them that sold and bought in the temple, and overthrew the tables of the moneychangers, and the seats of them that sold doves, And said unto them, It is written, My house shall be called the house of prayer; but ye have made it a den of thieves."
– Matthew 21:12-13
But basically any bank system would, I hope, try to appear more archaic in operation. The Vikings actually had a very robust trade system, so I wouldn't be surprised to find if they had money lenders and so forth. They would also chop coins into halves and fourths, which I think would make a lot of sense in game. For instance, a loaf of bread would cost a half or 4th a septim, and each loaf would actually constitute 3 items of "bread."