Who cried out against Jeroboam's altar at Bethel?

Answer: A man of God from Judah

A man of God from Judah cried out against the altar at Bethel, foretelling that a son of David named Josiah would burn human bones on it.

“Suddenly, as Jeroboam was standing beside the altar to burn incense, there came a man of God from Judah to Bethel by the word of the LORD.”— 1 Kings 13:1 (BSB)