A. From the law of God.
 Rom. 3: 20
For a scholarly treatment of the various uses of the law in reformed theology, please see Joel Beeke’s wonderful treatment in this article: http://www.ctsfw.edu/events/symposia/papers/sym2005beeke.pdf
The Heidelberg is here mentioning the “Evangelical” use of the law. Martin Luther called this use of the law as a “beating stick”. It batters you and beats you to the point of despair, where assurance can only come from God’s grace. After being beaten to Christ, the law then becomes a “walking stick” guiding us in sanctification.
Here are some basic summaries of the 3 uses of the law for the Christian from Beeke’s article:
“The Civil Use of the Law: The first use of the law is its function in public life as a guide to the civil magistrate in the prosecution of his task as the minister of God in things pertaining to the state. The magistrate is required to reward good and punish evil (Rom. 13:3-4). Nothing could be more essential to this work than a reliable standard of right and wrong, good and evil. No better standard can be found than the law of God.”
“The Evangelical Use of the Law: Wielded by the Spirit of God, the moral law also serves a critical function in the experience of conversion. It disciplines, educates, convicts, curses. The law not only exposes our sinfulness; it also condemns us, pronounces a curse upon us, declares us liable to the wrath of God and the torments of hell. “Cursed is every one that continueth not in all things which are written in the book of the law to do them” (Gal. 3:10). The law is a hard taskmaster; it knows no mercy. It terrifies us, strips us of all our righteousness, and drives us to the end of the law, Christ Jesus, who is our only acceptable righteousness with God. “Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith” (Gal. 3:24). Not that the law itself can lead us to a saving knowledge of God in Christ. Rather, the Holy Spirit uses the law as a mirror to show us our impotence and our guilt, to shut us up to hope in mercy alone, and to induce repentance, creating and sustaining the sense of spiritual need out of which faith in Christ is born.”
“The Didactic Use of the Law: The third or didactic use of the law addresses the daily life of the Christian. In the words of the Heidelberg Catechism, the law instructs the believer how to express gratitude to God for deliverance from all his sin and misery”