The order of the commandments in the Decalogue has its own inner logic that writers have described in various ways. Below I have translated Johann Gerhard’s discussion from the twelfth locus (On the Law of God) of his Loci theologici. This is followed by a long passage from Thomas Aquinas (Summa theologiae I-II, Q. 100, Art. 5) that gives Thomas’s account.
Gerhard on the Decalogue
To be sure, the harmonious order of the commandments of the Decalogue is a great proof of divine wisdom. The commandments of the first table treat of the immediate worship of God; the commandments of the second table treat of the mediate worship of God, since duties to one’s neighbor ought to proceed from true love and fear of God. In this way, they become worship that is pleasing and acceptable to God.
The first commandment takes its beginning from the most noble object, namely, the true God, and claims for itself the most noble subject, namely, the very heart of man, which it commands to cling to God with true fear, confidence, and love.
The second commandment forms the tongue of man, which ought to confess, call upon, glorify, and give thanks to God once he has been rightly known from his Word. After the heart, the tongue, the instrument of articulate speech, by which man is separated from the beasts, is man’s most noble member. This little bell ought so to ring as is drawn by the cord of the heart.
The third commandment institutes the sanctification of the Sabbath, that is, the public worship of God through the ministry of the Word, for by this means God wishes to kindle true knowledge and invocation of himself in man.
The second table, which treats of the duties owed to one’s neighbor, takes its beginning from the first natural society, which is that between parents and their children. The society of the household is the foundation and seed-bed of the commonwealth: therefore, by a fitting order, the fourth commandment, which is the first of the second table, treats of the honor owed to parents.
The fifth commandment guards man’s life and defends it from all injuries.
The sixth commandment preserves unharmed the honor of marriage.
The seventh commandment preserves property.
The eighth commandment sets up a blockade around man’s reputation with a strong wall and palisade.
The ninth commandment requires interior rectitude of the affections toward one’s neighbor.
The tenth commandment contains the general declaration of the law, namely, that in it is required an interior conformity to God of all one’s powers so perfect that it is not contaminated by even the smallest little stain of wicked concupiscence.