Albert Barnes Commentary 2 Chronicles 21:12

"And there came a writing to him from Elijah the prophet, saying, Thus saith Jehovah, the God of David thy father, Because thou hast not walked in the ways of Jehoshaphat thy father, nor in the ways of Asa king of Judah," — 2 Chronicles 21:12 (ASV)

This is the only mention of Elijah in Chronicles. As a prophet of the northern kingdom, he received little attention from the historian of the southern one. This reference shows that Elijah did not limit his attention to the affairs of his own state but also worked to stop the spread of idolatry in Judah. Furthermore, it proves that he was alive after the death of Jehoshaphat (2 Chronicles 21:13). This is a significant fact, for it:

1. Bears on the chronological order of 2 Kings 2:1 (see the note).
2. Shows that Elisha, who prophesied in the time of Jehoshaphat (2 Kings 3:11–19), began his public ministry before his master’s translation.