As I reflected on today's readings, I was really struck* by how simple the Gospel really is. In the readings today, Our Lord sums up the Gospel message in one sentence: “Repent, for the Kingdom of God is at hand.” (Mt 4:17) Likewise, St. John sums up how to live as the Gospel demands in one sentence in his First Letter: “we should believe in the name of his Son, Jesus Christ, and love one another just as he commanded us.” (1 John 3:23) That's it. Everything else that we believe as Christians builds on those two sentences. Our prayers, devotions, liturgies, doctrines, and dogmas come out of those brief statements.
So, why is the Catechism of the Catholic Church so big? Simple, we keep finding ways to screw up this simple message, and need to be corrected. If we all repented of our sins, did as Our Lord commanded, and loved our neighbors as ourselves, the Catechism would be about as long as this blog post. Since we keep trying to find loopholes or exceptions to the rule, we need more guidance. Hence the 900+ pages in the Catechism.
* NB: I never start a homily like this. It's bad homiletics, as it should be implied in your preparation. For a blog post, one needs to set the scene that brought up the post.