How did rulers use religion to support/justify their positions?

Generally speaking, rulers throughout history have used religion to add a touch of spiritual legitimacy to their rule. Think of it this way, if it's widely believed that a king or queen has been appointed by God, that makes it so much more difficult to challenge them. After all, in such a political environment, by challenging the ruler you're also challenging God, and that's not just treason, but blasphemy.

Explanation:

a king decides to declare war on a neighboring country without the input of any advisers