Here's why:
* Simile: A figure of speech that directly compares two things using words like "like" or "as."
* Metaphor: A figure of speech that implies a comparison between two things without using "like" or "as."
In this case, the sentence uses the word "in the midst," which is a phrase used to describe a situation where one thing is surrounded by others. This clearly establishes a comparison between "He" and "David" in the context of facing "many goliaths."
While the sentence doesn't explicitly use "like" or "as", the structure and meaning clearly indicate a comparison between "He" and David.