ANSWER: Since the fall, no mere human has been able to keep the law of God perfectly, but consistently breaks it in thought, word, and deed.
Catechism questions seven through twelve outlined God's law, and now question thirteen reinforces the awful truth: No one can keep this law perfectly. How does Paul describe our human condition in Romans 3:9-20? Why do you think he uses so many references from Psalms, Proverbs, Isaiah, and Ecclesiastes (see verses 10-18) to describe us? For some of the Old Testament references, you can look through Psalms 5, 10, 14, 36, 140; Proverbs 1; Isaiah 59; and Ecclesiastes 7.
The commentary on this catechism question comes from the Puritan John Owen, who wrote, "Never let us reckon that our work in contending against sin, in crucifying, mortifying, and subduing of it, is at an end." (Full comments are here; click on the "c:" icon.) In other words, even those who trust Jesus Christ to save them continue to struggle against sin for the time being. In Philippians 3:8-16 how does Paul tell us to live in this state of being simul iustus et peccator ("simultaneously just and sinner")?
As you meditate this week on the law of God and the death and resurrection of Christ, you might find helpful a poem by Jill Peláez Baumgaertner, an excerpt of which appears below. It is presented in full in Wheaton Magazine (see p. 27) for use during Advent but seems perfectly appropriate for Holy Week as well.
The Law sculpts our sin
in bas relief. We trace
its outline, rehearse
its shape, feel once again