See here,

I.How Rehoboam was strengthened by the accession of the priests and Levites, and all the devout and pious Israelites, to him, even all that were true to their God and their religion.

1.Jeroboam cast them off, that is, he set up such a way of worship as he knew they could not in conscience comply with, which obliged them to withdraw from his altar, and at the same time he would not allow them to go up to Jerusalem to worship at the altar there; so that he totally cast them off from executing the priest's office, Ch2 11:14. And very willing he was that they should turn themselves out of their places, that room might be made for those mean and scandalous persons whom he ordained priests for the high places, Ch2 11:15. Compare Kg1 12:31. No marvel if he that cast off God cast off his ministers; they were not for his purpose, would not do whatever he might bid them do, would not serve his gods, nor worship the golden image which he had set up.

2.They thereupon left their suburbs and possessions, Ch2 11:14. Out of the lot of each tribe the Levites had cities allowed them, where they were comfortable provided for and had opportunity of doing much good. But now they were driven out of all their cities except those in Judah and Benjamin. One would think their maintenance well settled, and yet they lost it. It was a comfort to them that the law so often reminded them that the Lord was their inheritance, and so they should find him when they were turned out of their house and possessions. But why did they leave their possessions? (1.) Because they saw they could do no good among their neighbours, in whom (now that Jeroboam set up his calves) the old proneness to idolatry revived. (2.) Because they themselves would be in continual temptation to some base compliances, and in danger of being drawn insensibly to that which was evil. If we pray, in sincerity, not to be led into temptation, we shall get and keep as far as we can out of the way of it. (3.) Because, if they retained their integrity, they had reason to expect persecution from Jeroboam and his sons. The priests they made for the devils would not let the Lord's priests be long among them. No secular advantages whatsoever should draw us thither, or detain us there, where we are in danger of making shipwreck of faith and a good conscience.

3.They came to Judah and Jerusalem (Ch2 11:14) and presented themselves to Rehoboam, Ch2 11:13, margin. Where should God's priests and Levites be, but where his altar was? Thither they came because it was their business to attend at the times appointed. (1.) It was a mercy to them that they had a place of refuge to flee to, and that when Jeroboam cast them off there were those so near that would entertain them, and bid them welcome, and they were not forced into the lands of the heathen. (2.) It was an evidence that they loved their work better than their maintenance, in that they left their suburbs and possessions in the country (where they might have lived at ease upon their own), because they were restrained from serving God there, and cast themselves upon God's providence and the charity of their brethren in coming to a place where they might have the free enjoyment of God's ordinances, according to his institution. Poverty in the way of duty is to be chosen rather than plenty in the way of sin. Better live upon alms, or die in a prison, with a good conscience, than roll in wealth and pleasure with a prostituted one. (3.) It was the wisdom and praise of Rehoboam and his people that they bade them welcome, though they crowded themselves perhaps to make room for them. Conscientious refugees will bring a blessing along with them to the countries that entertain them, as they leave a curse behind them with those that expel them. Open the gates, that the righteous nation, which keepeth truth, may enter in; it will be good policy. See Isa 26:1, Isa 26:2.

4.When the priests and Levites came to Jerusalem all the devout pious Israelites of every tribe followed them. Such as set their hearts to seek the Lord God of Israel, that made conscience of their duty to God and were sincere and resolute in it, left the inheritance of their fathers and went and took houses in Jerusalem, that they might have free access to the altar of God and be out of the temptation to worship the calves, Ch2 11:16. Note, (1.) That is best for us which is best for our souls; and, in all our choices, advantages for religion must take place of all outward conveniences. (2.) Where God's faithful priests are his faithful people should be. If Jeroboam cast off God's ministers, every true-born Israelite will think himself obliged to own them and stand by them. Forsake not the Levite, the out-cast Levite, as long as thou livest. When the ark removes do you remove and go after it, Jos 3:3.

5.They strengthened the kingdom of Judah (Ch2 11:17), not only by the addition of so many persons to it, who, it is likely, brought what they could of their effects with them, but by their piety and their prayers they procured a blessing upon the kingdom which was a sanctuary to them. See Zac 12:5. It is the interest of a nation to protect and encourage religion and religious people, and adds more than any thing to its strength. They made him and his people strong three years; for so long they walked in the way of David and Solomon, their good way. But when they forsook that, and so threw themselves out of God's favour and protection, the best friends they had could no longer help to strengthen them. We retain our strength while we cleave to God and our duty, and no longer.

II.How Rehoboam was weakened by indulging himself in his pleasures. He desired many wives, as his father did (Ch2 11:23), yet, 1. In this he was more wise than his father, that he does not appear to have married strange wives. The wives mentioned here were not only daughters of Israel, but of the family of David; one was a descendant from Eliab, David's brother (Ch2 11:18), another from Absalom, probably that Absalom who was David's son (Ch2 11:20), another from Jerimoth, David's son. 2. In this he was more happy than his father, that he had many sons and daughters; whereas we read not of more than one son that his father had. One can scarcely imagine that he had no more; but, if he had, they were not worth mentioning; whereas several of Rehoboam's sons are here named (Ch2 11:19, Ch2 11:20) as men of note, and such active men that he thought it his wisdom to disperse them throughout the countries of Judah and Benjamin (Ch2 11:23), either, (1.) That they might not be rivals with his son Abijah, whom he designed for his successor, or rather, (2.) Because he could repose a confidence in them for the preserving of the public peace and safety, could trust them with fenced cities, which he took care to have well victualled, that they might stand him in stead in case of an invasion. After-wisdom is better than none at all; nay, they say, "Wit is never good till it is bought;" though he was dearly bought with the loss of a kingdom.

Such as set their hearts to seek the Lord - All the truly pious joined him out of every tribe, and the whole tribe of Levi, being deprived of their functions, joined him also. Thus he had Judah, Benjamin, and Levi, and probably a part of Simeon; for he had Etam, which was in that tribe, and the truly religious out of all the other tribes, for they could not bear Jeroboam's idolatry.

And Rehoboam took him Mahalath, the daughter of Jerimoth, the son of David, to wife,.... Of which son of David we nowhere else read; perhaps he might be the son of one of his concubines, or he might have two names:

and Abihail the daughter of Eliab the son of Jesse; who was David's eldest brother. Sa1 17:13. She must be a granddaughter or great-granddaughter of his.

he ordained him priests--The persons he appointed to the priesthood were low and worthless creatures (Kg1 12:31; Kg1 13:33); any were consecrated who brought a bullock and seven rams (Ch2 13:9; Exo 29:37).

for the high places--Those favorite places of religious worship were encouraged throughout the country.

for the devils--a term sometimes used for idols in general (Lev 17:7). But here it is applied distinctively to the goat deities, which were probably worshipped chiefly in the northern parts of his kingdom, where the heathen Canaanites still abounded.

for the calves which he had made--figures of the ox gods Apis and Mnevis, with which Jeroboam's residence in Egypt had familiarized him. (See on Kg1 12:26).