Overview

Before expounding the Ten Commandments one by one, the catechism gives the interpretive keys: the law summarized in the Ten Commandments (Q98), the eight rules for rightly understanding them (Q99), what to consider in them (Q100), the preface (Q101), and the sum of the first table (Q102). Q99 is among the most useful passages in all the Standards for reading the commandments rightly.

The Questions & Answers

Q98. Where is the moral law summarily comprehended? A. The moral law is summarily comprehended in the ten commandments, which were delivered by the voice of God upon mount Sinai, and written by him in two tables of stone; and are recorded in the twentieth chapter of Exodus. The four first commandments containing our duty to God, and the other six our duty to man.

Scripture Proofs (ESV):

“And he declared to you his covenant… the Ten Commandments, and he wrote them on two tablets of stone.” (Deut. 4:13, ESV)

Further proofs: Exod. 20:1–17; Matt. 22:37–40.

Q99. What rules are to be observed for the right understanding of the ten commandments? A. [Summarized] (1) The law is perfect, binding everyone to full conformity in the whole man, and to perfect obedience forever. (2) It is spiritual, reaching the understanding, will, affections, and all the powers of the soul, as well as words, works, and gestures. (3) One and the same thing in divers respects is required or forbidden in several commandments. (4) As where a duty is commanded, the contrary sin is forbidden; and where a sin is forbidden, the contrary duty is commanded. (5) What God forbids is at no time to be done; what he commands is always our duty, yet every particular duty is not to be done at all times. (6) Under one sin or duty, all of the same kind are forbidden or commanded, together with all causes, means, occasions, and appearances thereof. (7) What is forbidden or commanded to ourselves, we are bound, according to our places, to endeavour that it may be avoided or performed by others. (8) In what is commanded to others, we are bound to be helpful to them, and to take heed of partaking with others in what is forbidden them.

Scripture Proofs (ESV):

“You therefore must be perfect, as your heavenly Father is perfect.” (Matt. 5:48, ESV)

“…the law is spiritual.” (Rom. 7:14; cf. Matt. 5:21–22, 27–28, ESV)

Further proofs: Ps. 19:7; Deut. 6:5; Luke 16:10; Col. 3:5; 1 Tim. 5:22.

Q101. What is the preface to the ten commandments? A. The preface to the ten commandments is contained in these words, I am the Lord thy God, which have brought thee out of the land of Egypt, out of the house of bondage. Wherein God manifesteth his sovereignty, as being JEHOVAH, the eternal, immutable, and almighty God; having his being in and of himself, and giving being to all his words and works: and that he is a God in covenant, as with Israel of old, so with all his people; who, as he brought them out of their bondage in Egypt, so he delivereth us from our spiritual thraldom; and that therefore we are bound to take him for our God alone, and to keep all his commandments.

Scripture Proofs (ESV):

“I am the LORD your God, who brought you out of the land of Egypt, out of the house of slavery.” (Exod. 20:2, ESV)

Further proofs: Luke 1:74–75; 1 Pet. 1:15–18; Lev. 18:30.

Exposition: Q99 is the master key to the whole Decalogue, and the most important to grasp. Its eight rules teach us that the commandments are perfect (rule 1) and spiritual (rule 2) — reaching the heart, not just outward acts, exactly as Jesus taught (lust is adultery, anger is murder). Rule 4 is especially fruitful: every command implies its opposite — “Thou shalt not kill” commands us to preserve life; “Thou shalt not steal” commands generosity. Rule 6 extends each command to “all causes, means, occasions, and appearances” of the sin or duty. Rules 7–8 make us responsible for others’ obedience, not just our own. These rules transform the Ten Commandments from a short list of prohibitions into a comprehensive guide to holy living of the whole person. Q101’s exposition of the preface roots all obedience in grace: God is the covenant Redeemer who delivered His people before commanding them — so we obey out of redemption, not for it.

Connections

• WSC Q41–44 — the moral law summarized in the Ten Commandments; the sum of the law; the preface.
• WCF 19 (Of the Law of God).
• Loci: The Decalogue; the spirituality and extent of the law; the hermeneutics of the commandments.

Pastoral & Practical Significance

Q99’s eight rules are perhaps the most practically transformative passage in the catechisms, because they teach us how to read God’s commands so that no one can hide behind mere external compliance. The law is spiritual (rule 2): it judges the heart’s thoughts, desires, and affections, not just outward deeds — which means we are all far guiltier than a surface reading suggests, and so are driven again to Christ. The law is positive as well as negative (rule 4): each “Thou shalt not” contains a “Thou shalt,” so the commandments call us not merely to avoid evil but to pursue active good — to preserve life, promote truth, further our neighbor’s welfare. And each command sweeps in “all causes, means, occasions, and appearances” (rule 6), so we are to flee not only sin itself but everything that leads to it. Far from making obedience easier, these rules make the law inescapable — which is precisely their grace, for they leave us no refuge but Christ, and then, in Christ, they give us a rich and comprehensive vision of the holy life. And Q101 ensures we never read the commands apart from the gospel: the God who commands is the God who first redeemed.

For Memory & Reflection

• Study Q99 carefully — it is the key to rightly understanding every commandment. Memorize Q101 (the preface); anchor verses: Matthew 5:48 and Exodus 20:2.
• The law is “spiritual,” reaching the heart. How does rule 2 expose sins a surface reading would miss?
• Each command implies its opposite duty (rule 4). Take one commandment and ask: what positive good does it require of me?