Bernius Madolffo was a good and honest priest who turned to entrepreneurship after the dissolution of the Holy Roman Empire. The death of the Catholic Church and fall of Christianity in the world during the plague actually turned out to be a positive thing in Europe, as it spread chaos and allowed it to fall into warring factions each vying for control of the continent. As any rational businessman would tell you, chaos is a ladder, and the absence of a corrupt and hyprocritical church created an atmosphere of suffering for a good and honest man to profit.
Madolffo's plan is to established a World Bank in Europe that would loan money to rival kingdoms to support their military and nation building adventures. Want to fund a large project such as a great wall or palace in your land and lack the ability to pay? The bank would give you a loan to solve your problems. Want to fund an expedition to find new trade routes abroad, or an expedition to a foreign land to genocide the indigenous population in order to establish a colony? The bank would have your back and loan you capital. These loans would come with a 15% - 35% interest rate, burdening the nation with debt that would take centuries to pay off. Bernius's ultimate goal is to spread the bank's influence to the seven kingdoms (Africa, Asia, Europe, Australia, North and South America, and Antarctica) and unite them under a cycle of debt. While leader play their wargames with each other, it will be the bank that holds the strings and profit from it.
However, this plan comes with risk. Nations are always at risk of not cooperating. Some possess massive armies due to their population and may decide not to pay what they owe. If they lose a war, the conquering enemy will ignore the bills of their defeated foe. Nations may simply decide that the bank is a threat because of funding their enemies, and decide to attack the bank directly, destroying it in the process. All of these scenarios result in the banking industry losing money on their investments. The knights Templar also had a banking system, and we know what happened to them. The king didn't want to pay, and had them executed for heresy.
Madolffo always pays his dental, but some of his clients would not. How can Madolffo establish this banking system while preserving it from potential opportunists long term?