The Church authorities (notably the Pope) held more power than kings or queen.
The Catholic Church was very rich and powerful during the Middle Ages. Eventually, the church owned about one third of the land in Western Europe, and because the church was considered independent, they did not have to pay the king any tax for their land.
For all the wealth they accumulated, the Church paid no taxes, which gave them economic power surpassing even the wealth of some monarchs. Bishops and Clergy owed their allegiance not to their King but to the Pope in Rome which led to more than a few conflicts over the years.
Another effect of the spread of Islam was an increase in trade. Unlike early Christianity, Muslims were not reluctant to engage in trade and profit; Muhammad himself was a merchant. As new areas were drawn into the orbit of Islamic civilization, the new religion provided merchants with a safe context for trade.
