"There is more than enough in the catechism for a lifetime. One may have mastered the very words of the catechism, but still the catechism has more to teach when one is confronted by difficulties and deep truths of the Bible: If even the disciples doubted Jesus’ resurrection, how does the church still exist? How is the Old Testament useful and profitable for Christians? What are ghosts? The same old catechism shines new light on new questions. That’s why Luther said so often that he might be an old, learned doctor of the Bible, but all the same daily he must humble himself and pray the catechism side-by-side with the little children.
This should not be surprising. For Luther, the catechism is nothing other than the Bible itself. And the Bible is like a mysterious body of water which sheep can wade into and drink, while an elephant can drown in the same water. The Bible might look like any other book, but it cannot be read like any other book. Any other book can eventually be mastered—all of its knowledge and secrets mapped and categorized. But not the Bible! “Christians understand the word of God—they can even talk about it, but they can never be done learning it. ”As the psalmist says: “His wisdom is not to be measured” (Ps 147:5)."