We need the context. It sounds like students doing homework or adults in a contest-type reality show.
"one with Christ imagery" - a video that contains symbols or images of Jesus Christ.
"we can totally justify skipping English" - we can miss the English class without being punished.
Adventure Time is a TV cartoon series.
"Free-response essay" - known as "an open question" in the UK. Its opposite is "a multiple choice test". Respondents or students are given a topic and a target number of words; they express their views on the topic, and support their views with data and logical arguments.
The following free-response questions, for example, will lend themselves to an essay using Adventure Time as an answer:
"If you were a parent and you were to watch a cartoon series with your child, which series would you choose? Explain and justify your choice."
"If you were an advertisement salesman, and you were responsible for recruiting advertising clients for Adventure Time in a national channel, who would those clients be? What arguments, data and images would you include in your sales pitch? Analyse your persuasion techniques."
As to why discovery of imagery of Christ would open the door to using Adventure Time as the subject of or material for an essay, we need more context.