Q: Where is the moral law summarily comprehended?
A: The moral law is summarily comprehended in the ten commandments.
Deuteronomy 4:13: And he declared to you his covenant, which he commanded you to perform, that is, the Ten Commandments, and he wrote them on two tablets of stone.
I think it is an important distinction as we move into the catechism's section on the Ten Commandments, that the commandments are viewed as a summary of the moral law. The Ten Commandments cover every sin imaginable, but they do this by painting with broad strokes. They are not only addressing outward actions (as I think the Scribes and Pharisees understood them), but also the underlining heart issues of a person. Jesus shows us this in Matthew 5 in the Sermon on the Mount. He himself shows how the commandments are a summary. "You have heard it said, 'Thou shall not murder'...But I say to you that everyone who is angry with his brother will be made liable to judgment..." or, "You have heard it said, 'Thou shall not commit adultery'...But I say to you whoever looks at a woman with lustful intent has already committed adultery with her in his heart..." Christ was not adding to the Ten Commandments, rather he was getting to the core of what the commandments were summarizing. The catechism, just as Christ did in his teachings, will begin to expand upon each commandment and show us how comprehensive the Ten Commandments really are. When we begin to understand all that the Ten Commandments are summarizing and addressing, then we begin to understand how deep our sin truly goes, then we will understand all the more just how dependent we truly are on those indicatives in our desires and attempts to uphold and keep the imperatives of God's law!