Albert Barnes Commentary 1 Kings 18:25

"And Elijah said unto the prophets of Baal, Choose you one bullock for yourselves, and dress it first; for ye are many; and call on the name of your god, but put no fire under." — 1 Kings 18:25 (ASV)

Elijah gives precedence in everything to the Baal-priests, to remove any basis for complaint in case of failure. His objective is to make an impression on the king and people, and he rightly understands that the impression will depend greatly on the contrast between their inability and the power given to him.