From John Flavel’s An Exposition of the Assembly’s Shorter Catechism:
Q. 6. What is the first reason annexed to the second commandment?
A. The first reason annexed is God’s sovereignty, I the Lord; which shews that it belongs to God only to institute his own worship, and make it effectual; and therefore to do that in his worship which he never commanded, is sinful and dangerous; Jeremiah 7:31. And they have built the high places of Tophet, which is in the valley of the son of Hinnom, to burn their sons and their daughters in the fire, which I commanded them not, neither came it into my heart.
Q. 7. What is the second reason annexed to the second commandment?
A. The second reason is God’s propriety in us: He is our God, and we belong to him; and therefore to corrupt his worship, greatly aggravates our sins; Hosea 9:1. Rejoice not, O Israel, for joy, as other people; for thou hast gone a whoring from thy God, &c.
Q. 8. What is the third reason annexed to the second commandment?
A. The jealousy of God over his worship and worshippers; so that this sin of corrupting his worship will dreadfully incense his wrath, as it did, Leviticus 10:1-2. And Nadab and Abihu, the sons of Aaron, took either of them his censer, and put fire thereon, and offered strange fire, before the Lord, which he commanded them not. And there went out fire from the Lord, and devoured them, and they died before the Lord.
Q. 9. What is the first instruction from the second commandment?
A. That it is an heinous sin to neglect the worship of God in that manner he hath appointed us to worship him, as in prayer; Jeremiah 10:25. Pour out thy fury upon the heathen that know thee not, and upon the families that call not on thy name. Hearing the word; Proverbs 28:9. He that turneth away his ear from hearing the law, even his prayer shall be abomination.
Q. 10. What is the second instruction from the second commandment?
A. That those who suffer for endeavouring to preserve the purity of God’s ordinances, and nonconformity to the contrary injunctions of men, have a good warrant to bear them out in all such sufferings; Deuteronomy 4:2. Ye shall not add unto the word which I command you, neither shall you diminish ought from it, that ye may keep the commandments of the Lord your God, which I command you.
Q. 11. What is the third instruction from the second commandment?
A. That it is highly sinful and dangerous to innovate and prescribe by human authority such symbolical rites in the worship of God, as he never appointed or allowed in his word; Matthew 15:9. But in vain do they worship me, teaching for doctrines the commandments of men.
Q. 12. What is the fourth instruction hence?
A. Hence we learn how much parents and children are obliged to worship God constantly, spiritually, and agreeably to his will revealed in his word; otherwise the jealousy of God will visit them both in the way of judgment: For as obedience entails a blessing, so disobedience entails a curse on posterity; Exodus 34:14. For thou shalt worship no other god; for the Lord, whose name is Jealous, is a jealous God.
Exclusive Psalmody 