Albert Barnes Commentary 1 Kings 12:30

"And this thing became a sin; for the people went [to worship] before the one, even unto Dan." — 1 Kings 12:30 (ASV)

This thing became a sin—that is, this act of Jeroboam’s became an occasion of sin for the people. The author suggests the original text may have read as follows: The people went to worship before the one in Bethel and the other in Dan.