While it is completely appropriate to draw on someone else's findings and conclusions in your work, it is punishable to present them as your own. Plagiarism happens when we use someone else's ideas without listing that individual as a source. It is considered as a serious breach of academic ethics.
It is not allowed to use other people's ideas without citing them, but it is also punishable to mix your own previous work with new without permission. This is known as <em>self-plagiarism</em>.
It's a metaphor because it's "a thing regarded as representative or symbolic of something else, especially something abstract." (which is the definition of a metaphor) They are using the "Man named Latimer.." quote as a metaphor to the above thing they mentioned about God.
