literally
Explanation
The Joke
The comic opens with a "fun fact" stating that a decent number of Biblical literalists have not actually read the Bible, and that this presents an opportunity. A man then tests this by asking someone if they really believe that "Obadiah rode a 3-headed land-shark into battle against the Edomites" -- a completely fabricated event that is not in the Bible at all. The Biblical literalist, who has clearly not read the source material, responds with firm conviction: "I do, sir, and you will not lead me astray."
The Humor
The humor comes from the irony of someone who claims to take the Bible literally and refuses to be "led astray" from its teachings, while simultaneously demonstrating that they have no idea what is actually in the text. The invented scenario of Obadiah riding a three-headed land-shark is deliberately absurd, making the man's earnest acceptance of it all the funnier. The comic points out a real cultural phenomenon where people claim authority from a text they have not read, and suggests that their commitment is more to the identity of being a literalist than to the actual content of the literature.
References
Obadiah is a real book of the Bible -- one of the shortest books in the Old Testament, consisting of a single chapter about the judgment of Edom. It contains no land-sharks of any number of heads. The Edomites are also a real Biblical people, which makes the fabrication slightly more plausible to someone unfamiliar with the details.