In 12:25-33 and 13:33-34 we have an account of the apostasy of Jeroboam.
Jeroboam made two calves of gold, putting one in Bethel and the other in Dan, in order to distract his people from worshipping God in Jerusalem (12:25-30). God had ordained that His people come together three times a year in Jerusalem. Jeroboam was afraid that the ten tribes would return to their rightful king if they went to worship God in Jerusalem. Thus, he set up two worship centers, saying that it was not convenient to travel to Jerusalem. The excuse of convenience also is used to justify today's denominations. Jeroboam's apostasy broke God's ordination of having one unique worship center in the holy land for keeping the unity, the oneness, of the children of Israel (Deut. 12:2-18). This became a great sin and caused the people to worship idols.
Jeroboam built a temple at the high places and appointed priests from among the common people who were not of the tribe of Levi (1 Kings 12:31). Whereas God had ordained that the tribe of Levi would be the priests, Jeroboam appointed common people to be priests.
Jeroboam ordained a feast in the eighth month, on the fifteenth day of the month, like the feast that was in Judah (v. 32a), doing what he had devised in his own heart. He might have told the people that it was not necessary for them to go to Jerusalem in order to have a feast.
Jeroboam offered sacrifices on the altar at Bethel to the calves that he had made, and he placed in Bethel the priests of the high places (vv. 32b-33a).
In his apostasy Jeroboam ordained a feast for the children of Israel, and, although he was not a priest, he went up to the altar to burn incense (v. 33b).