I wouldn't say religious power is nothing compared to the power of a king, but I would say religious power can be used as another form of political power, and often is. One advantage religion has over politics is its direct access to an emotional hold over its professers, which gives it more buttons to push where it comes to social control. The medieval Roman Catholic church had great political power because of its strong following among the common people - it was able to inspire fear and loyalty in a way few kings could because of what it represented.
Many of the "evils of religion" - though not all - are in reality politically motivated.
My guess would be that people who blame everything horrible on religion only have a superficial understanding of history. The core of many religions is a striving for betterment, and most people who would call themselves religious just want to be good people. But as with every organisation that has ever existed since we came down out of the trees (or whatever we did), the cake of religion is flavoured with a liberal sprinkling of idiots. So is the cake of Atheism.
