During the 19th century, philosophers applied the concept of evolution to the emerging science of comparative religion. They believed, that, like creation, religion moved from the simple to the complex. They believed Elohim was evidence of the polytheistic stage in Judaism. There is, of course, no polytheism in Judaism or Christianity. Some think God used this plural name for Himself in a kingly way.
Kings often refer to themselves in the plural form to emphasize their authority.
But this theory has little biblical evidence to support it. Instead, many interpreters assume God is referring to the triune aspect of the Godhead, to the Trinity. Other interpreters think that God is referring to His many-faceted characteristics, which have unity in the one. God has many attributes, but they are all perfectly unified. God is not just holy, His love is holy, His wrath is holy, etc.