There is no reasonable doubt that this "book of the law" was the Pentateuch - nearly, if not quite, in the shape in which we now have it. Copies of the whole Law were, no doubt, scarce; and therefore Jehoshaphat's commission took care to carry a copy with them.

Had the book of the law of the Lord with them - This was their text book: it was the book of God; they taught it as such, and as such the people received it. Its laws were God's laws, and the people felt their obligation, and their consciences were bound. Thus they were obedient to the laws of the land, on the principle of religion. In this they were encouraged and confirmed by the example of all, both in Church and state. The princes were not only pious, but were teachers of piety; the Levites showed them the worth and excellence of their ritual institutions; and the priests showed them the moral use they were to make of the whole: and thus the people became obedient to God as well as to the king, and kept all the civil ordinances, not merely for the sake of a good king, but for the sake of a good and gracious God. By these means the nation enjoyed peace and prosperity; and all insurrections, seditions, and popular commotions, were prevented. The surrounding nations, perceiving this, saw that there was no hope of subduing such a people, so they made no war with Jehoshaphat, 2 Chronicles 17:10. And they took care not to provoke such a people to fall on them; therefore, it is said, The fear of the Lord fell on all the kingdoms and lands that were round about Judah. Such an itinerant ministry established in these kingdoms for upwards of fourscore years, teaching the pure, unadulterated doctrines of the Gospel, with the propriety and necessity of obedience to the laws, has been the principal means, in the hand of God, of preserving these lands from those convulsions and revolutions that have ruined and nearly dissolved the European continent. The itinerant ministry, to which this refers, is that which was established in these lands by the late truly reverend, highly learned and cultivated, deeply pious and loyal John Wesley, A.M., formerly a fellow of Lincoln College, Oxford, whose followers are known by the name of Methodists; a people who are an honor to their country, and a blessing to the government under which they live.

And they taught in Judah,.... The people their duty to God and man:

and had the book of the law of the Lord with them; as the rule of their instruction, and the people's obedience; out of this they taught them, and by it confirmed what they taught, and enforced it with the divine authority:

and went about throughout all the cities of Judah; not only their own cities, the Levitical ones, but all others:

and taught the people; the law of the Lord, the fear and worship of God.

And they taught in Judah, and had the book of the law of the LORD with them, and went about throughout all the cities of Judah, and taught the people.

17:9 And they taught, and c. - And these itinerant judges and itinerant preachers together, Mr. Henry observes were instrumental to diffuse a blessed light throughout the cities of Judah.

9. the book of the law-that is, either the whole Pentateuch or only the book of Deuteronomy, which contains an abridgment of it.

17:1-19 Jehoshaphat promotes religion in Judah, His prosperity. - Jehoshaphat found his people generally very ignorant, and therefore endeavoured to have them well taught. The public teaching of the word of God forms, in all ages, the great method of promoting the power of godliness. Thereby the understanding is informed, the conscience is awakened and directed. We have a particular account of Jehoshaphat's prosperity. But it was not his formidable army that restrained the neighbouring nations from attempting any thing against Israel, but the fear of God which fell upon them, when Jehoshaphat reformed his country, and set up a preaching ministry in it. The ordinances of God are more the strength and safety of a kingdom, than soldiers and weapons of war. The Bible requires use to notice the hand of God in every event, yet this is little regarded. But let all employ the talents they have: be faithful, even in that which is little. Set up the worship of God in your houses. The charge of a family is important. Why should you not instruct them as Jehoshaphat did his subjects, in the book of the law of the Lord. But be consistent. Do not recommend one thing, and practise another. Begin with yourselves. Seek to the Lord God of Israel, then call upon children and servants to follow your example.