Despite the influx of Levites and other godly people from northern Israel, Rehoboam abandoned God and led Judah into sin:
2 Chronicles 12:1 But when Rehoboam was firmly established and strong, he abandoned the Law of the Lord, and all Israel followed him in this sin.
Their sin, we’re told, was even worse than that of their ancestors:
1 Kings 14:23-24For they also built for themselves pagan shrines and set up sacred pillars and Asherah poles on every high hill and under every green tree. There were even male and female shrine prostitutes throughout the land. The people imitated the detestable practices of the pagan nations the Lord had driven from the land ahead of the Israelites.
How did it get to this?
Rehoboam was at least partly responsible, since he had installed his sons as rulers over the fortified towns in Judah and looked after all their material needs:
2 Chronicles 11:3 Rehoboam also wisely gave responsibilities to his other sons and stationed some of them in the fortified towns throughout the land of Judah and Benjamin. He provided them with generous provisions, and he found many wives for them.
But their leadership appears to have not gone beyond the material comforts, since they fled to Jerusalem for safety when Shishak invaded.
Even so, it wasn’t all bad:
2 Chronicles 11:12 Because Rehoboam humbled himself, the Lord’s anger was turned away, and he did not destroy him completely. There were still some good things in the land of Judah.