Catechism
Explanation
The Joke
A person writes in to "Dear God" saying they have been reading the Westminster Shorter Catechism and have questions. They acknowledge the questions may seem harsh, but note they were "made by people who love you" and they just want to put them at the end of their letter to God. The page then turns to show a spotlight on an envelope labeled "The Dear God" -- building dramatic tension -- and then we see the questions amount to nothing but uproarious laughter: "HAHAHAHAHAHAHAHAHA HAHAHAHAHAHAHA!"
The implication is that the questions in the Westminster Shorter Catechism are so absurd, so presumptuous, or so hopelessly naive when directed at an actual omniscient deity that God's only possible response is to laugh hysterically rather than dignify them with answers.
The Humor
The comic works on multiple levels. On one hand, it pokes fun at the earnest theological tradition of catechisms -- structured sets of questions and answers meant to teach religious doctrine -- by suggesting that if God actually received these questions, the deity would find them laughably inadequate or silly. On another level, it plays with the classic "Dear God" letter format that children often use, juxtaposing that innocent framing with the weighty, centuries-old Westminster Shorter Catechism. The punchline of pure laughter is effective because it leaves the specific reason for God's amusement entirely to the reader's imagination, which is funnier than any explicit answer could be.
References
The Westminster Shorter Catechism is a Reformed Christian catechism written in the 1640s by the Westminster Assembly. Its most famous question is Q1: "What is the chief end of man?" with the answer: "Man's chief end is to glorify God, and to enjoy him forever." It contains 107 questions and answers covering theology, the Ten Commandments, the Lord's Prayer, and the sacraments.