A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Chronicles 15:1
ch2 15:1JUDAH MAKES A SOLEMN COVENANT WITH GOD. (Ch2 15:1-15)
Azariah the son of Oded--This prophet, who is mentioned nowhere else, appears at this stage of the sacred story in the discharge of an interesting mission. He went to meet Asa, as he was returning from his victorious pursuit of the Ethiopians, and the congratulatory address here recorded was publicly made to the king in presence of his army.
2 Chronicles 15:2
ch2 15:2The Lord is with you, while ye be with him--You have had, in your recent signal success, a remarkable proof that God's blessing is upon you; your victory has been the reward of your faith and piety. If you steadfastly adhere to the cause of God, you may expect a continuance of His favor; but if you abandon it, you will soon reap the bitter fruits of apostasy.
2 Chronicles 15:3
ch2 15:3Now for a long season Israel hath been without the true God, &c.--Some think that Azariah was referring to the sad and disastrous condition to which superstition and idolatry had brought the neighboring kingdom of Israel. His words should rather be taken in a wider sense, for it seems manifest that the prophet had his eye upon many periods in the national history, when the people were in the state described--a state of spiritual destitution and ignorance--and exhibited its natural result as widespread anarchy, mutual dissension among the tribes, and general suffering (Jdg 9:23; Jdg 12:4; Jdg 20:21; Ch2 13:17). These calamities God permitted to befall them as the punishment of their apostasy. Azariah's object in these remarks was to establish the truth of his counsel (Ch2 15:2), threatening, in case of neglecting it by describing the uniform course of the divine procedure towards Israel, as shown in all periods of their history. Then after this appeal to national experience, he concluded with an earnest exhortation to the king to prosecute the work of reformation so well begun [Ch2 15:7].
2 Chronicles 15:7
ch2 15:7Be ye strong--Great resolution and indomitable energy would be required to persevere in the face of the opposition your reforming measures will encounter.
your work shall be rewarded--What you do in the cause and for the glory of God will assuredly be followed by the happiest results both to yourself and your subjects.
2 Chronicles 15:8
ch2 15:8when Asa heard . . . the prophecy of Oded the prophet--The insertion of these words, "of Oded the prophet," is generally regarded as a corruption of the text. "The sole remedy is to erase them. They are, probably, the remains of a note, which crept in from the margin into the text" [BERTHEAU].
he took courage--Animated by the seasonable and pious address of Azariah, Asa became a more zealous reformer than ever, employing all his royal authority and influence to extirpate every vestige of idolatry from the land.
and out of the cities which he had taken from mount Ephraim--He may have acquired cities of Ephraim, the conquest of which is not recorded (Ch2 17:2); but it has been commonly supposed that the reference is to cities which his father Abijah had taken in that quarter (Ch2 13:19).
renewed the altar of the Lord . . . before the porch--that is, the altar of burnt offering. As this was done on or about the fifteenth year of the reign of this pious king, the renewal must have consisted in some splendid repairs or embellishments, which made it look like a new dedication, or in a reconstruction of a temporary altar, like that of Solomon (Ch2 7:7), for extraordinary sacrifices to be offered on an approaching occasion.
2 Chronicles 15:9
ch2 15:9he gathered all Judah and Benjamin--Not satisfied with these minor measures of purification and improvement, Asa meditated a grand scheme which was to pledge his whole kingdom to complete the work of reformation, and with this in view he waited for a general assembly of the people.
and the strangers with them out of Ephraim and Manasseh--The population of Asa's kingdom had been vastly increased by the continued influx of strangers, who, prompted by motives either of interest or of piety, sought in his dominions that security and freedom which they could not enjoy amid the complicated troubles which distracted Israel.
and out of Simeon--Although a portion of that tribe, located within the territory of Judah, were already subjects of the southern kingdom, the general body of the Simeonites had joined in forming the northern kingdom of Israel. But many of them now returned of their own accord.
2 Chronicles 15:10
ch2 15:10the third month--when was held the feast of pentecost. On this occasion, it was celebrated at Jerusalem by an extraordinary sacrifice of seven hundred oxen and seven thousand sheep, the spoil of the Ethiopians being offered. The assembled worshippers entered with great and holy enthusiasm into a national covenant "to seek the Lord their God . . . with all their heart and with all their soul;" and, at the same time, to execute with rigor the laws which made idolatry punishable with death (Ch2 15:13; Deu 17:2-5; Heb 10:28). The people testified unbounded satisfaction with this important religious movement, and its moral influence was seen in the promotion of piety, order, and tranquillity throughout the land.
2 Chronicles 15:18
ch2 15:18the things that his father had dedicated--probably part of the booty obtained by his signal victory over Jeroboam, but which, though dedicated, had hitherto been unrepresented.
and that he himself had dedicated--of the booty taken from the Ethiopians. Both of these were now deposited in the temple as votive offerings to Him whose right hand and holy arm had given them the victory.