This thing became a sin - i. e., this act of Jeroboam’s became an occasion of sin to the people. The author perhaps wrote the following words thus: “The people went to worship before the one to Bethel and before the other to Dan.”
Related Commentaries of 1 Kings 12:30
1 Kings 12:30
30 And this thing became a sin: for the people went to worship before the one, even unto Dan.