We really need both.
I was reminded of this last week when a friend challenged me on the Small Catechism meaning of the 8th commandment. He couldn't see how the commandment had anything to do with the meaning. The Large Catechism gives more of the thought process why "Do not bear false witness" means uphold and defend your neighbor's good name and reputation. That got me musing on how the Small Catechism was written for father's to use in teaching their children the truths of Scripture. It lays a good foundation, but doesn't necessarily produce maturity. The Large Catechism grapples with deeper issues and goes into more depth. It's not suited for the minds of children or spiritual babes.
Unfortunately, too many of the Lutherans sitting in our pews are satisfied that they were forced to memorize parts of the Small Catechism in Confirmation and see no need to seek spiritual maturity. I've been preaching through the Ten Commandments for Lent, using the Small Catechism meanings as a rough outline and drawing heavily on the Large Catechism and on Luther's Treatise on Good Works. This has been eye opening for many members who haven't really thought about the issues raised in the Commandments since Confirmation. I had the same reaction when I preached through the Apostle's Creed two years ago.
So, we need both. The Small Catechism to lay the foundations of faith and the Large Catechism to draw people to spiritual maturity.
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He is intelligent, but not experienced. His pattern indicates two-dimensional thinking. - Mr. Spock